The Baltimore Catechism
366. Q. What is the Fifth Commandment?
A. The Fifth Commandment is: Thou shalt not kill.
A. We are commanded by the Fifth Commandment to live in peace and union with our neighbor, to respect his rights, to seek his spiritual and bodily welfare, and to take proper care of our own life and health.
“Proper care of our own life.” It is not our property, but God’s. He lends it to us and leaves it with us as long as He pleases: nor does He tell us how long He will let us have the use of it. Thus suicide, or the taking of one’s own life, is a mortal sin, for by it we resist the will of God. One who in sound mind and full possession of reason causes his own death is guilty of suicide. But it is sometimes very difficult to determine whether the person was really sane at the time he committed the act; hence, when there is any reasonable doubt on that point, the unfortunate suicide is usually given the benefit of it. It is also a sin to risk our lives uselessly or to continue in any habit that we are sure is injuring our health and shortening our lives.
Thus an habitual drunkard is guilty of sin against the Fifth Commandment, for besides his sin of drunkenness, he is hastening his own death. So, too, boys or girls who indulge in habits which their parents forbid are guilty of sin. For example, a boy is forbidden to smoke, and he does smoke. Now to smoke is not in itself a sin, but it becomes a sin for that boy, because in the first place he is disobedient, and secondly is injuring his health. Thus persons who indulge in sinful habits may commit more than one kind of sin, for besides the sins committed by the habits themselves, these vices may injure their health and bring sickness and disease upon their bodies.
A. The Fifth Commandment forbids all willful murder, fighting, anger, hatred, revenge, and bad example.
Therefore it forbids all that might lead to murder. So we can violate any of the Commandments by doing anything that leads to breaking them. “Revenge” is a desire to injure others because they injured you.