The fifth Commandment actually uses the word “murder,” which is rasah in Hebrew, phoneuo in Greek, and occides in Latin as found in original Biblical manuscripts.
The word “kill” would be harag in Hebrew, apokteino in Greek, or interficias in Latin. This means that the commandment literally says, “Thou shalt not murder” and not, “Thou shalt not kill.” Murder is the deliberate killing of an innocent person. Self-defense is not considered murder, and killing in self-defense is neither sinful nor immoral.
Murder can be premeditated (first degree in civil law), spontaneous but deliberate deadly assault (second degree), or it can be an unlawful killing of a human being without malice (manslaughter). These are all forbidden by the fifth Commandment.
Equivalent acts would also include any and all unjust killing, such as abortion or euthanasia, where the death is directly intended and achieved—regardless of the motive or consequences. Intentionally ending the life of an innocent person is always considered unjust killing and is immoral and sinful. Gross acts would be genocide, terrorism, and unjust wars, or indiscriminate use of deadly force.
Extended acts would include physical violence or abuse, verbal or emotional abuse, harboring hateful thoughts, racism, bullying, intimidation, and blackmail. Jesus said in the Gospel of Matthew 5:21–22 that he who is angry with his brother is guilty of murder.
What Christ is speaking of is the willingness to commit murder. The desire to murder someone is a sinful act of the will. Let’s say I buy a gun and plan to kill my annoying neighbor, but someone or something prevents me from leaving the house to get to his home or, when the moment arrives and I pull the trigger, I discover there are no bullets in the gun, or that I missed the target, or that I left the safety on by mistake so the gun does not fire.
All these obstacles may prevent the physical act of murder, but I have already committed the sinful desire to murder when I took the gun in my hand. I committed murder in the heart if not in the flesh. My act of the will was that I commit murder.