This is often explored with Batman, who is a “Technical Pacifist”. Some disagree with his “no killing” code, but Batman inevitably brings up the point that “will it stop at one”? You kill the Joker, who is an obvious deranged psychopath. Do you then kill Bane, a murderer who is tactical mastermind with drug-enhanced strength? Do you kill Falcone, whose nothing more than a crime boss? Do you kill Dent, who may not even be culpable for his actions due to a clear case of mental instability and/or multiple personalities? Do you kill Kite-Man, roped into a bad situation due to his engineering skills?
Once the line is crossed, the descent to darkness often begins. For this reason, among others, I usually side with “the end never justifies the means”.