The guy is dead.
I think to my self, is it wrong to kill a killer? Would history be different if the guy was killed in the 1980's? Would lives have been saved? Most people agree that if an attacker is trying to kill another person that it is acceptable to use deadly force to stop the attacker and save life. I do not see this kind of thing any different. The man may not have been driving a car or pointing a gun, but he was systematically killing children and doing so with the blessing of the state.
Something is very wrong when we say that it is morally wrong to put an end to the life of a killer especially one that the authorities and defenders refuse to stop. It's not wrong to kill, killing and murder are different. That man was guilt of murder. If someone would have killed him then morally speaking they would not have been guilty of murder because they were doing it in defense of life regardless of what the law says.
This is not a confession of what I intend to do, I have too much to lose and so will not do it, but it does not mean I'm not allowed to speak and spark the idea in someone elses heart. And who knows, if I'm last alive in my clan then maybe I will pick up the sword of justice and swing it to save life. And if I am fallen while saving life, then I am a hero even if no one else knows it and that is good enough for me.