Homicide

[nomercy]

The only moral justification for killing a person is direct self-defense. When a criminal is confined in prison, he does not threaten society, so killing him is not justified by self-defense. And even if capital punishment deters others from committing similar crimes, this would be an indirect defense against murder rather than directly from the criminal being killed. Direct self-defense implies killing someone who himself, at that time, is a threat to other persons, and this precludes taking human life just for punishment.

Full article here: http://www.progress.org/fold56.htm

So far I haven't found anything on the legalities of murder as self-defence.