I believe the death penalty is necessary because some people do not deserve to live. Why can we justify killing people by shooting them as a necessary barbaric

I believe the death penalty is necessary because some people do not deserve to live. Why can we justify killing people by shooting them as a necessary barbaric act during war, but not recognize that the worst criminals deserve the same treatment? It's absurd to think that as a society we are above killing because it's an inevitable part of life. I say murderers should not be able to kill others without having to fear that they themselves will be put to death.