Personally, I have absolutely no problem with karma and "justice" being helped along by people that have got the correct person. Personally, I don't think it is wrong to be satisfied if a someone kills a burglar or rapist, a person kills another that was attacking them, etc. What person would say that it wouldn't be a relief that the bad guy was gone? I don't think it is wrong for someone to kill a person attacking innocent people, period.
I believe that at some point in our lives, we will get what we give. Some people learn from their mistakes others don't and incur increasing damage upon the communities in which they reside or pass through. Unfortunately, if not for good guys killing the correct bad guys, some bad guys actually would get away with their criminal actions and only go elsewhere to commit more. It's always nice to see this process stopped in it's tracks by any means.