There are some cases where revenge killings have been accepted as mitigating circumstances, but it is always on a case by case basis.
You have, for example, the Don Ayala-Paula LLoyd case.
Paula LLoyd was a anthropologist who worked as a volunteer in Afghanistan. In november 2008, she was part of a team visiting a village, she went to talk to a vilager who was holding a can of gasoline. For an unknown reason, he doused her with gasoline and set her on fire. She suffered first/second degree burns to 60% of her body and died two months later.
Don Ayala who was on her team immediately detained the attacker who was then held by other soldiers. Ayala went to check on the condition of LLoyd. When he came back 10 minutes later, he shot and killed the attacker with one bullet to the head. At that point, the attacker was lying on the ground with his hands tied behind his back.
Ayala was initially charged with 2nd degree murder, but he eventually pleaded guilty to manslaughter and was only sentenced to probation.
Obviously an extreme case.
http://www.nbcnews.com/id/30645926/