Q. 1. What is the fifth Commandment and what are the sins against it?
A. 1. The fifth Commandment is, “You shall not kill.“
"Human life is sacred because from its beginning it involves the creative action of God and it remains for ever 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." [CDF, instruction, Donum vitae, intro. 5] (C.C.C. # 2258)
What is forbidden by this Commandment?
It is forbidden unjustly to take the life of a human being. It is forbidden to take the life of an unborn child. Hatred, anger, jealousy, quarreling, excessive drinking are forbidden, because these may lead to killing, or may injure ourselves and others.
"All who hate a brother or sister are murderers, and you know that murderers do not have eternal life abiding in them.” [1 Jn. 3:15] “You have heard that it was said to those of ancient times, ‘You shall not murder’; and 'whoever murders shall be liable to judgment.’ But I say to you that if you are angry with a brother or sister, you will be liable to judgment." [Mt. 5:21-2] (C.C.C. # 2054)
Since the first century the Church has affirmed the moral evil of every procured abortion. This teaching has not changed and remains unchangeable. Direct abortion, that is to say, abortion willed either as an end or a means, is gravely contrary to the moral law. (C.C.C. # 2271)
Those whose lives are diminished or weakened deserve special respect. Sick or handicapped persons should be helped to lead lives as normal as possible. (C.C.C. # 2276)
Whatever its motives and means, direct euthanasia consists in putting an end to the lives of handicapped, sick, or dying persons. It is morally unacceptable. Thus an act or omission which, of itself or by intention, causes death in order to eliminate suffering constitutes a murder gravely contrary to the dignity of the human person and to the respect due to the living God, his Creator. The error of judgment into which one can fall in good faith does not change the nature of this murderous act, which must always be forbidden and excluded. (C.C.C. # 2277)
Intentional euthanasia, whatever its forms or motives, is murder. It is gravely contrary to the dignity of the human person and to the respect due to the living God, his Creator. (C.C.C. # 2324)
When is it permitted to take the life of another person?
If lawful authority commands it in punishment for graves crimes. Although statistics have proven that the application of the death penalty does not serve as a deterrent to crimes. In a just war. A "just war" is normally viewed as defending one's nation against an invasion. Invading another nation is rarely considered as a "just war." In self-defense in order to save one’s life, or to save the life of another who is unjustly attacked.
Is it permitted to take one’s own life? No. Suicide is a grave sin. We have no dominion over our own life. 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 ties 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. (C.C.C. # 2281)
Note: Many "Living Will" are classified as suicide. It is forbidden for Catholics to write a Living Will that would authorize the medical profession to end one's life by deprivation of the basic needs such as oxygen, medication(s), food and water due to a person's temporary or permanent medical condition.