1. “You shall not kill"
“ Human life is sacred because from its beginning it involves the creative action of God and it remains forever in a special relationship with the Creator, who is its sole end. God alone is the Lord of life from its beginning until its end: no one can under any circumstance claim for himself the right directly to destroy an innocent human being" (Catechism 2258).
Man is unique, the only creature on earth that God has loved for its own sake.  Destined to know and love God eternally, man's life is sacred. All men and women are created in the image and likeness of God (cf. Gen 1:26-27). This is the ultimate basis of human dignity and of the commandment not to kill.
The book of Genesis presents the assault on human life as the consequence of original sin. Yahweh always shows himself to be the protector of life, even of Cain's life after he kills his brother Abel, blood of his own blood (the image of every homicide). No one should take justice into his own hands, and no one can give himself the right to dispose of his neighbor's life (cf. Gen 4:13-15).
This commandment applies to human beings. It is permissible to make use of animals for food, clothing, etc. God put them on earth to be useful for man. The requirement not to kill or mistreat them comes from the disorder human passions may involve, or from a duty of justice (if they are someone else's property) (cf. Catechism 2418). Besides, it must not be forgotten that man is not the “master" of creation, but rather its administrator, and as such is obliged to respect and care for nature, which he needs for his own existence and development (cf. Catechism 2415).
2. Deepest element of this commandment
“The deepest element of God's commandment to protect human life is the requirement to show reverence and love for every person and the life of every person." 
Mercy and pardon are proper to God. Mercy must also be present in the life of a child of God, stirring up in our heart compassion for those who are suffering: Blessed are the merciful, for they shall obtain mercy (Mt 5:7). 
It is also necessary to learn to forgive those who offend us (cf. Mt 5:22). When others offend us, we must try not to lose our peace, and never let anger gain control of our heart. Moreover, in the Our Father , the prayer that Jesus left us as the Lord's prayer, Christ links his forgiveness—pardon for the transgressions we have committed—to our pardon for those who have offended us (cf. Mt 6:9-13; Lk 11:2-4). We will be helped in this struggle by the contemplation of the passion of our Lord, who pardoned and redeemed us by bearing with love and patience his unjust treatment. We should also keep in mind that a Christian should never see anyone as an alien or an enemy (cf. Mt 5:44-45), and that we will be judged after death according to our love for our neighbor. Christians are asked to overcome evil with good (cf. Rom 12, 21), and to see insults as an opportunity for our own purification.
3. Respect for human life
The fifth precept orders us “not to kill." It also prohibits striking, hurting or causing unjust bodily pain to oneself or to another, as well as insulting our neighbor with offensive words or wishing evil on him. This commandment also includes the prohibition of killing oneself (suicide).
3.1 Intentional homicide
“The fifth commandment forbids direct and intentional killing as gravely sinful. The murderer and those who cooperate voluntarily in murder commit a sin that cries out to heaven for vengeance" (Catechism 2268). 
The encyclical Evangelium vitae has set forth in a definitive and infallible way the following norm: “by the authority which Christ conferred upon Peter and his Successors, and in communion with the Bishops of the Catholic Church, I confirm that the direct and voluntary killing of an innocent human being is always gravely immoral. This doctrine, based upon that unwritten law which man, in the light of reason, finds in his own heart (cf. Rom 2:14-15), is reaffirmed by Sacred Scripture, transmitted by the Tradition of the Church and taught by the ordinary and universal Magisterium."  Thus homicide is without exception gravely immoral if it is an act resulting from a deliberate decision and directed to an innocent person . As such, legitimate defense and the death penalty are not included in this definitive formulation, and are dealt with in a separate section. 
The fact that God has entrusted human life into our hands calls for an attitude of love and service, and not of arbitrary dominion. God alone is the Master of human life. 
“Human life must be respected and protected absolutely from the moment of conception. From the first moment of his existence, a human being must be recognized as having the rights of a person" (Catechism 2270).
“ Direct abortion, that is, abortion willed as an end or as a means, always constitutes a grave moral disorder, since it is the deliberate killing of an innocent human being."  “No circumstance, no purpose, no law whatsoever can ever make licit an act which is intrinsically illicit, since it is contrary to the Law of God which is written in every human heart, knowable by reason itself, and proclaimed by the Church."  The respect for life must be recognized as the boundary that no activity on the part of an individual or a government can cross. The inalienable right of every innocent human person to life is a constitutive element of a civil society and its legislation and as such must be respected by both society as a whole and the political authority (cf. Catechism 2273). 
“The right to command is required by the moral order and has its source in God; it follows that if civil authority legislate for or allow anything that is contrary to that order and therefore contrary to the will of God, neither the laws made nor the authorizations granted can be binding on the consciences of the citizens … Otherwise, authority breaks down completely and results in shameful abuse."  So much so that “there is no obligation in conscience to obey such laws; instead there is a grave and clear obligation to oppose them by conscientious objection ." 
“Since it must be treated from conception as a person, the embryo must be defended in its integrity, cared for, and healed, as far as possible like any other human being" ( Catechism 2274).
“ Euthanasia in the strict sense is understood to be an action or omission which of itself and by intention causes death, with the purpose of eliminating all suffering … euthanasia is a grave violation of the law of God since it is the deliberate and morally unacceptable killing of a human person … Depending on the circumstances, this practice involves the malice proper to suicide or murder."  Gravely opposed to the dignity of the human person, euthanasia is one of the consequences that hedonism and the loss of the Christian sense of suffering can lead to.
“Discontinuing medical procedures that are burdensome, dangerous, extraordinary, or disproportionate to the expected outcome can be legitimate; it is the refusal of 'over-zealous' treatment. Here one does not will to cause death; one's inability to impede it is merely accepted" (Catechism 2278). 
Nevertheless, “even if death is thought imminent, the ordinary care owed to a sick person cannot be legitimately interrupted" (Catechism 2279).  Artificial nutrition and hydration are, in principle, ordinary care owed to all the sick. 
“We are stewards, not owners, of the life God has entrusted to us. It is not ours to dispose of" (Catechism 2280). “Suicide contradicts the natural inclination of the human being to preserve and perpetuate his life. It is gravely contrary to the just love of self. It likewise offends love of neighbor because it unjustly breaks the rites of solidarity with family, nation, and other human societies to which we continue to have obligations. Suicide is contrary to love for the living God" (Catechism 2281). 
To choose to die to save the life of someone else is not suicide; on the contrary, it can be an act of the utmost charity.
3.5. Legitimate defense
The prohibition against causing anyone's death does not suppress the right to prevent an unjust aggressor from causing injury.  Legitimate defense can also be a grave duty for someone responsible for the life of another person or for the common good (cf. Catechism 2265).
3.6. The death penalty
Defending the common good requires rendering an aggressor unable to do harm. Therefore, the legitimate authority can impose punishment commensurate with the gravity of the crime committed. The aim of punishments is to compensate for the disorder caused by the offense, to ensure public order and the security of persons, as well as the correction of the guilty person (cf. Catechism 2266). “For these purposes to be achieved, the nature and extent of the punishment must be carefully evaluated and decided upon, and ought not go to the extreme of executing the offender except in cases of absolute necessity: in other words, when it would not be possible otherwise to defend society … such cases are very rare, if not practically non-existent." 
4. Respect for the dignity of the human person
4.1. Respect for the soul of one's neighbor: scandal
Christians have the duty to safeguard the life and supernatural health of their neighbor's soul and body.
In contrast, “scandal is an attitude or behavior which leads another to do evil. The person who gives scandal becomes his neighbor's tempter … Scandal is a grave offense if by deed or omission another is deliberately led into a grave offense" ( Catechism 2284). Scandal can be caused by unjust comments, by promoting immoral shows, books and magazines, by immodest dress, etc.
“Scandal takes on a particular gravity by reason of the authority of those who cause it or the weakness of those who are scandalized" (Catechism 2285). Whoever causes one of these little ones who believe in me to sin, it would be better for him to have a great millstone fastened round his neck and to be drowned in the depth of the sea (Mt 18:6). 
4.2. Respect for the body's health
Respect for one's own body is a requirement of charity since the body is the temple of the Holy Spirit (cf. 1 Cor 6:19; 3, 16ff; 2 Cor 6:16), and we are responsible, insofar as it depends on us, to maintain our health as a means of serving God and others. However, bodily life is not an absolute value. Christian morality is opposed to a neo-pagan conception that fosters the cult of the body and can lead to perverting human relationships (cf. Catechism 2289).
“The virtue of temperance disposes us to avoid every kind of excess: the abuse of food, alcohol, tobacco, or medicine. Those incur grave guilt who, by drunkenness or a love of speed, endanger their own and others' safety on the road, at sea, or in the air." (Catechism 2290).
The use of drugs is a grave offense because of the harm it can do to one's health and the loss of responsibility in acts accomplished under their influence. Clandestine production and traffic of drugs are immoral activities (cf. Catechism 2291).
Scientific research cannot justify acts that, in themselves, are contrary to the dignity of the human person and the moral law. No human being can be treated as a means for the progress of science (cf. Catechism 2295). This principle is transgressed by practices of substitute artificial procreation or the use of embryos for experimental purposes.
4.3. Organ transplants
Giving organs as transplants is licit and can be an act of charity if the donation is without constraint and free,  and if it is arranged in a just and charitable way.
“A person can donate an organ only if it involves no serious danger of harm for his own life or personal identity, and for a just and proportionate reason. As an obvious consequence, vital organs can be donated only after death." 
It is necessary for the donor or his representatives to give their informed consent (cf. Catechism 2296). This donation, “although licit in itself, can become illicit if it violates the rights and feelings of third parties responsible for guardianship of the body: close relatives in the first place; but this may also include other persons by virtue of public or private rights." 
4.4. Respect for physical freedom and for bodily integrity
Kidnapping and hostage taking are morally illicit. They involve treating persons merely as means to obtaining some end, depriving them unjustly of their freedom. Likewise, terrorism and torture gravely violate justice and charity.
“Except when performed for strictly therapeutic medical reasons, directly intended amputations, mutilations, and sterilizations performed on innocent persons are against the moral law" (Catechism 2297).
4.5. Respect for the dead
“The bodies of the dead must be treated with respect and charity, in faith and hope of the Resurrection. The burial of the dead is a corporal work of mercy (cf. Tob 1:6-18); it honors the children of God, who are temples of the Holy Spirit" (Catechism 2300). “The Church strongly advises maintaining the pious custom of burying the body of the deceased; however it does not forbid cremation, provided it has not been chosen for reasons contrary to Christian doctrine" (CIC, can. 1176).
5. Safeguarding peace
Blessed are the peacemakers for they shall be called sons of God (Mt 5:9). A characteristic of the spirit of divine filiation is to be sowers of peace and joy .  “Peace cannot be attained on earth without safeguarding the goods of persons, free communication among men, respect for the dignity of persons and peoples, and the assiduous practice of fraternity … Peace is the work of justice and the effect of charity" (Catechism 2304).
“Because of the evils and injustices that accompany all war, the Church insistently urges everyone to prayer and to action so that the divine Goodness may free us from the ancient bondage of war (cf. Vatican Council II, Const. Gaudium et spes, 81,4)" (Catechism 2307).
There can be a need for “legitimate defense by military force." However “the gravity of such a decision makes it subject to rigorous conditions of moral legitimacy" (Catechism 2309). 
“Injustice, excessive economic or social inequalities, envy, distrust, and pride raging among men and nations constantly threaten peace and cause wars. Everything done to overcome these disorders contributes to building up peace and avoiding war" (Catechism 2317).
“Love your own country: it is a Christian virtue to be patriotic. But if patriotism becomes nationalism, which leads you to look at other people, at other countries, with indifference, with scorn, without Christian charity and justice, then it is a sin." 
Pau Agulles Simo
Catechism of the Catholic Church , 2258-2330.
John Paul II, Enc. Evangelium vitae , 25 March 1995, ch. 3.
 Cf. Vatican Council II, Const. Gaudium et spes , 24.
 John Paul II, Enc. Evangelium vitae , 25 March 1995, 41.
 “The works of mercy are charitable actions by which we come to the aid of our neighbor in his spiritual and bodily necessities" ( Catechism 2447).
 Furthermore, it “forbids doing anything with the intention of indirectly bringing about a person's death. The moral law prohibits exposing someone to mortal danger without grave reason, as well as refusing assistance to a person in danger" ( Catechism 2469).
 John Paul II, Evangelium vitae , 57.
 Cf. Ibid , 55-56.
 Cf. Ibid , 52.
 Ibid , 62.
 Ibid , 62. The crime of abortion is of such gravity that the Church sanctions this transgression with the canonical punishment of excommunication latae sententiae (cf. Catechism 2272).
 “These human rights depend neither on single individuals nor on parents; nor do they represent a concession made by society and the State: they pertain to human nature and are inherent in the person by virtue of the creative act from which the person took his of her origin … the moment a positive law deprives a category of human beings of the protection which civil legislation must accord them, the State is denying the equality of all before the law. When the State does not place its power at the service of the rights of each citizen, and in particular of the more vulnerable, the very foundations of a State based on law are undermined" (Congregation for the Doctrine of the Faith, Instr. Donum vitae , 22 February 1987, III).
“How many crimes are committed in the name of justice! If you were a dealer in guns and someone paid you the price of one of them, so that he might use the weapon to kill your mother, would you sell it to him? And yet, wasn't he ready to pay you a just price for it? Professor, journalist, politician, diplomat: reflect" (St Josemaria, The Way, 400).
 John XXIII, Enc. Pacem in terris , 11 April 1963, 51.
 John Paul II, Enc. Evangelium vitae , 73.
 John Paul II, Enc . Evangelium vitae , 65.
 “The decisions should be made by the patient if he is competent and able or, if not, by those legally entitled to act for the patient, whose reasonable will and legitimate interests must always be respected" (Catechism 2278).
 “The use of painkillers to alleviate the sufferings of the dying, even at the risk of shortening their days, can be morally in conformity with human dignity if death is not willed as either an end or a means, but only foreseen and tolerated as inevitable. Palliative care is a special form of disinterested charity. As such it should be encouraged" ( Catechism 2279).
 Cf. John Paul II, Discorso ai partecipanti al Congresso Internazionale su “I trattamenti di sostegno vitale e lo stato vegetativo. Progressi scientifici e dilemmi etici," 20 March 2004, no. 4; cf. Congregation for the Doctrine of the Faith, “Responses to Certain Questions of the United States Conference of Catholic Bishops Regarding Artificial Nutrition and Hydration," 1 August 2007.
 “We should not despair of the eternal salvation of persons who have taken their own lives. By ways known to him alone, God can provide the opportunity for salutary repentance. The Church prays for persons who have taken their own lives" ( Catechism 2283).
 “Love toward oneself remains a fundamental principle of morality. Therefore it is legitimate to insist on respect for one's own right to life. Someone who defends his life is not guilty of murder even if he is forced to deal his aggressor a lethal blow" ( Catechism 2264; cf. John Paul II, Enc. Evangelium vitae , 55). In this case, killing the aggressor does not constitute the direct purpose of the will of the person defending himself; rather, the moral aim consists in removing an imminent threat to one's own life.
 John Paul II, Enc. Evangelium vitae , 56. Cf. Catechism , 2267.
 “They are guilty of scandal who establish laws or social structures leading to the decline of morals and the corruption of religious practice, or to 'social conditions that, intentionally or not, make Christian conduct and obedience to the Commandments difficult and practically impossible' (Pius XII, Discourse , June 1, 1941)" ( Catechism 2286).
 Cf. John Paul II, Discourse , 22 June 1991, 3; Catechism , 2301.
 Ibid. 4.
 Pius XII, Discorso all'Associazione Italiana Donatori di Cornea , 14 May 1956.
 Cf. St Josemaria Christ is Passing By , 124.
 “At one and the same time:
—the damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain;
—all other means of putting an end to it must have been shown to be impractical or ineffective;
—there must be serious prospects of success;
—the use of arms must not produce evils and disorders graver than the evil to be eliminated. The power of modern means of destruction weighs very heavily in evaluating this condition.
“These are the traditional elements enumerated in what is called the 'just war' doctrine. The evaluation of these conditions for moral legitimacy belongs to the prudential judgment of those who have responsibility for the common good" (Catechism 2309). Moreover, “one is morally bound to resist orders that command genocide" (Catechism 2313).
Regarding the so-called arms race : “Far from eliminating the causes of war, it risks aggravating them. Spending enormous sums to produce ever new types of weapons impedes efforts to aid needy populations; it thwarts the development of peoples" ( Catechism 2315). The arms race “is an utterly treacherous trap for humanity, and one which injures the poor to an intolerable degree" (Vatican Council II, Const. Gaudium et spes , 81). Public authorities have the right and the duty to regulate the production and sale of arms (cf. Catechism 2316).
 St Josemaria, Furrow , 315. Cf. St Josemaria The Forge , 879; The Way , 525.