Would it then be acceptable to kill the killer of the murderers as he is now a murderer him self?
The act of killing someone is not murder, its just an act, murder is what we deem as killing someone with an inadequate reason.
Killing someone is justified if, you're protecting someone elses or your own life, it isn't murder. Killing people at war has rarely been considered murder, the reason the act is commited is more important than the act itself, and its always been that way.
If a vigilante started picking off murderers who got away with what they did, its a fine line, with some of them they WILL be preventing further violent crimes, which while not as specific as say shooting someone whose actually standing over someone with a knife and mid way through the motion to stab them, is still rather loosely self defence.
But this is where the lines blur, it will be difficult to know who will reoffend.
THe guys the op mentioned, the two kids who got 4 years, as nasty as the act was, they in no way meant to kill the guy, however it was directly their actions that led to his death, its not murder, its accidental manslaughter basically with a healthy amount of bad luck thrown in. I think they should have gotten a little longer, but considering they most certainly did not go out there to kill that guy that night, should they really have gotten the same sentence as someone who went and shot a guy in the head with the only intent being to end his life? FOr instance, someone mentioned Dexter, who generally goes after people who get away AND show no remorse and will almost certainly reoffend, these two kids didn't mean to kill someone, and will likely never do anything that could accidentally cause a death again as well, they never tried to kill anyone in the first place. The papers haven't covered it, but the two kids could be incredibly remorseful and sorry for what they did, or they could think its funny, who knows, we certainly don't.
As I pointed out earlier, the end result of someone being dead isn't anywhere near as significant as the reasoning, someone whose stupid actions results in an accidental death without intent should NOT be punished with the same sentence as someone who specifically and purposefully kills someone.