I don't think anyone should be killed in revenge for something they've done. There's too much going on inside a person's brain that we can't understand to call ourselves justified in ending their life.
"Eye for an eye" might've seemed like wisdom to the god-fearing shepherds of yesteryear, but I can't buy it.
On the other hand, if a person is a threat, then that threat has to be dealt with. If there's no reason to believe a person won't kill again, locking him away where he can't hurt anyone makes sense to me. Yes, it curtails his life, but there's a practicality that comes into play.
On the third hand, if a person is an immediate threat, then all bets are off. I wouldn't hesitate for a moment to use deadly force to prevent one person from harming another. Steal all the cars they want, vandalize to their heart's content -- they'll lose no blood at my hand. But threaten another person's life or limb and I'd have zero compunctions about stopping them, even at the expense of their life.
I've never killed anyone, so I have no idea how I'd feel afterward. But I'm pretty sure I'd do it.
Dave