From all the American documentaries I've seen regarding cases where various criminals have been shot dead by members of the public, one thing has always been constant, which is that you are only allowed to shoot to kill if, in your judgement, you are about to be killed or seriously injured, or raped.
In a case in Ohio a group of young people were throwing tomatoes at passing traffic, one irate driver pulled up, took his 12 guage shotgun and shot one young man to death.
Knowing full well that he had committed murder he left the scene and was arrested in due course when the cops tracked him down.
The jury decided that he was only guilty of a misdemeanor, and the judge sentenced him to time served (5 months)
http://www.daytondailynews.com/localnews/content/localnews/daily/0307weaver.html