A non-criminal homicide, usually committed in self-defense or in defense of another, may be called justifiable homicide in some cases. A homicide may be considered justified if it is done to prevent a very serious crime, such as rape, armed robbery, or murder. The assailant's intent to commit a serious crime must be clear at the time. A homicide performed out of vengeance, or retribution for action in the past would generally not be considered justifiable, although in some cases such a crime is classed as being justifiable due to the impossibility of finding a jury who would convict under the case's circumstances.

In cases of self-defense, there is generally a duty for the defendant to retreat if possible to do so (except from one's home or place of business), or it is not justifiable (in the state of Louisiana, there is no duty to retreat). Pre-emptive self-defense, cases in which one kills another because they suspect the victim might eventually become dangerous, is considered criminal, no matter how likely it is that they were right. Justifiable homicides are always initially assumed to be criminal until the evidence warrants a change, as justifiable homicide is one of the most common defenses for homicides both justified and criminal. Justifiable homicide is a legal grey area, and there is no real legal standard for a homicide to be considered justifiable. The circumstances under which homicide is justified are usually considered to be that the victim was clearly likely to kill a third person if the defendant did not kill them.

Another form of justifiable homicide is unique to the prison system. The death penalty and shooting escaping prisoners. To quote the California State Penal Code (state law) that covers justifiable homicide:

196. Homicide is justifiable when committed by public officers and those acting by their command in their aid and assistance, either--

1. In obedience to any judgment of a competent Court; or,

2. When necessarily committed in overcoming actual resistance to the execution of some legal process, or in the discharge of any other legal duty; or,

3. When necessarily committed in retaking felons who have been rescued or have escaped, or when necessarily committed in arresting persons charged with felony, and who are fleeing from justice or resisting such arrest.

Although the above text is from California USA, most states or countries have laws much the same to prevent escapes from custody.