Since life imprisonment is a form of torture that is worse than death in my opinion, I agree the death penalty isn't much of a deterrent. But it is a preventative. Someone in prison for life has nothing to lose and is a danger to other inmates as well as guards, since anything he does has no consequences and he knows it. If he is executed, this threat is removed; and since he is a known murderer, this threat is very real. Morally of course, the criminal has, by his own act, forfeited his right to his life by the act of taking another's, so it is not the government which is taking his life, it is the criminal himself. The government is simply enforcing the consequences of the criminal's chosen course of action. If the criminal knows he forfeits his life by taking another's, and does it anyway, then it is he who has made the decision to end his own life. What happens once he is caught is not a decision by the government, it is just the continuation of the chain of consequences the criminal set in motion himself.