The Fifth Commandment is: "Thou shalt not kill." The Fifth Commandment obliges us to take proper care of our own health and to live in peace and charity with our neighbors. By the Fifth Commandment we are obliged to love our neighbor as ourselves. As the catechism tells us: Our neighbor is all mankind without any exception of persons - even those who injure us or differ from us in religion. Jesus Christ tells us we must love not only our friends but also our enemies: "I say to you, love your enemies: do good to those that hate you: and pray for them that persecute and calumniate you. .....For if you love them that love you, what reward shall you have?.....And if you salute your brethren only, what do you more? Do not also the heathens do this." (Matt. 5:44-47).
Therefore we sin against this precept of brotherly love if we refuse to show our neighbor the ordinary signs of good will, such as refusing to speak to him or to return his salute. The sins against the Fifth Commandment are murder, suicide, mutilation, drunkenness, envy, revenge and also scandal. In three cases it is lawful to take the life of another: (a) In defense of one's life; (b) When a criminal is convicted and sentenced to death the State may take his life. But revenge for crime on private authority is never lawful. (c) In a just war. As it is impossible for the individual to know, in most cases, whether the war is just or unjust. in practice all may fight for their country, when it is at war.
The "conscientious objectors" who base their refusal to fight on the words "Thou shalt not kill," seem to forget that the Old Testament gives innumerable cases in which the chosen people of God fought in just wars against their enemies. Outside the cases given above no man has the right to take his own life or that of his neighbors. God alone is the master of life and death. That refers to human life unborn, from the very beginning and to life in its weakened and crippled stages. Those who would attack the unborn and those who would practice euthanasia, by putting an end to the life of an old man, or an incurable invalid, are alike murderers and they bear the mark of Cain. Murder is always a mortal sin and one of the sins that cry to Heaven for vengeance.
Serious mutilation of oneself or another without a justifying cause (like sterilization as means of birth control) is a mortal sin. Thus any surgeon would sin gravely who would remove any important organ of the human body without a sufficient cause.
Drunkenness is a mortal sin if it is complete and deprives the person of consciousness. The same must be said of the unnecessary use of drugs.
Quarreling, fighting, hatred, envy, jealousy, anger, or revenge can be mortal or venial sins according to the seriousness of the matter.
Scandal is causing another to sin. "Scandalum" means a stumbling block. So the scandal giver is like a stumbling block in the path of his neighbor. If by our bad example in word or deed we lead another into sin we kill or at least injure his soul. Scandal is a sin against this Commandment, as it attacks the life, not of the body, but of the soul which is immeasurably more precious. "Woe to that man by whom the sandal cometh," says Our Blessed Lord. "It were better for him that a millstone should be hanged about his neck and that he should be drowned in the depths of the sea." (Matt. 18:6.) All those who lead others into sin by word or deed or example or tho consent to sin with others are guilty of scandal. This sin is particularly serious in the case of parents: as it is their duty to teach their children to avoid sin and to love and serve God.
Source: Rev. W. Frean, C.SS.R. Commentary on the Catechism.
The Killing of Animals
Some have said that it is not permitted to kill even brute animals. But this is false, because it is not a sin to use that which is subordinate to the power of man. It is in the natural order that plants nourish animals, certain animals nourish others, and all nourish man: "Even as the green herbs have I delivered them all to you." (Gen,9:3) The Philosopher (Aristotle) says that hunting is like a just war. And St. Paul says: "Whatsoever is sold in the meat market eat, asking no questions for conscience sake." (1 Cor. 10:25.) Therefore, the sense of the Commandment is: "Thou shalt not kill men."
Anger is considered a passion. This is in the sensitive appetite and is twofold. Sometimes it is ordered by reason or restrained within proper limits by reason, as when a person is angry because it is justly fitting to be angry and within proper limits. This is an act of virtue and is called "righteous anger." Thus, the Philosopher says that meekness is not opposed to anger. This kind of anger, then, is not a sin.
Source: St. Thomas Aquinas, God's Greatest Gifts.