It's a commandment Christians were given, not me.
Killing the killer does nothing except kill them, it settles nothing, it resolves nothing, it won't even deter future murders but encourage killing witnesses. It's not justice, it's vengeance. The victims have already been victimized, relived it, testified in court, and killing someone else doesn't help them. But it does cause someone else to lose a daughter, father, aunt, or uncle. Losing a loved one is never fixed by making someone else lose a loved one.