This may be considered a part of the defense of provocation against charges of murder. Based on the idea that all individuals may suddenly and unexpectedly lose control when words are spoken or events occur, jurisdictions differ on whether this should be allowed to excuse liability or merely mitigate to a lesser offense such as manslaughter, and under which circumstances this defense can be used. In many common law jurisdictions, provocation is a partial defense that converts what would have been murder into manslaughter. A few jurisdictions do not prosecute (Iran, Iraq) or have a lesser penalty (Kuwait, Egypt) for honor killings. European Convention on Human Rights
Article 2 Paragraph 2 of the European Convention On Human Rights provides that death resulted from defending oneself or others, arresting a suspect or fugitive, or suppressing riots or insurrections, will not contravene the Article when the use of force involved is "no more than absolutely necessary":

2. Deprivation of life shall not be regarded as inflicted in contravention of this Article when it results from the use of force which is no more than absolutely necessary:
(a) in defence of any person from unlawful violence;
(b) in order to effect a lawful arrest or to prevent the escape of a person lawfully detained;
(c) in action lawfully taken for the purpose of quelling a riot or insurrection.
This has now been amended by §7 Judicial Matters Second Amendment Act 122 of 1998:
(2) If any arrestor attempts to arrest a suspect and the suspect resists the attempt, or flees, or resists the attempt and flees, when it is clear that an attempt to arrest him or her is being made, and the suspect cannot be arrested without the use of force, the arrestor may, in order to effect the arrest, use such force as may be reasonably necessary and proportional in the circumstances to overcome resistance or to prevent the suspect from fleeing: Provided that the arrester is justified in terms of this section in using deadly force that is intended or is likely to cause death or grievous bodily harm to a suspect, only if he believes on reasonable grounds-
(a) that the force is immediately necessary for the purpose of protecting the arrestor, any person lawfully assisting the arrestor or any other person from imminent or future death or grievous bodily harm;
(b) that there is a substantial risk that the suspect will cause imminent or future death or grievous bodily harm if the arrest is delayed; or
(c) that the offence for which the arrest is sought is in progress and is of a forcible and serious nature and involves the use of life threatening violence or a strong likelihood that it will cause grievous bodily harm. Circumstances which justify homicide in the United States
A non-criminal homicide ruling, usually committed in self-defense or in defense of another, exists under United States law.