StinkyPantz, I think your post consists of two questions.
Everyone so far has said that yes they would kill in order to protect family/friends. So I wonder, if you saw a stranger being murdered and you had the opportunity to stop their death by killing the perpetrator would you?
What if they were killing a family member?
I am assuming that in either situation, your hypothetical victim is known to be an entirely innocent party who has been randomly attacked by a complete stranger, which would make it a rather unusual case of murder.
By "innocent" I mean, this is not a case of Lyin' Eyes catching up with someone who has hurt her children!
I also assume that the ONLY way to prevent the murder would be to kill the attacker. It is a convoluted and highly unlikely hypothetical situation. Even adding in my assumptions, I don't know what I would do. I suppose this is the sort of situation that an armed officer of the law has to face at times. I guess, if I KNOW that the victim is entirely innocent, and I can simply and easily prevent the attack ONLY by killing the attacker, I would.
However, I am an individual, and your scenario deals with saving the life of the victim. Legislated execution by the state is a different thing, and in any case does not save the life of the victim - whatever happens to the perpetrator, the victim is still dead.