In extreme cases I'm all for taking all their organs and donating them to people who need them. Those may turn out to be killers, cure cancer or live normal/simple lives.
Bottom line, for me killing them the way we do now or keeping them in prison where they can have a decent live and corrupt others (or even kill more) is wrong and needs to change. They took lives so they should at least try to repay their dept to society by saving some lives in return.