How is the death penalty worse than a lifetime of imprisonment? If you kill them it's over and you killed them back case closed but, the have the oppertunity for eternal bliss still based on the final judgement of him, right? Improbable maybe but, a chance non the less. What else?
Lifetime in prison gives the person limited to no freedoms, dependent on their behavior in prison, a life time of guilt, if capable of feeling it or if they do regret the crime, a chance to redeem themselves to the family/friends of the victim, be found not guilty and set free, write a book, make music or do work for the prison.
In both cases they won't be able to hurt the public.
So, it's just the value of existence vs freedom with morals thrown in.