Murdering a mass murder is not stooping to the same level as the criminal, I think that's a very stupid argument that gets thrown around a lot.
The criminal murders the innocent, more often than not unprovoked. The state isn't senselessly murdering the innocent. The death penalty is used only on the most extreme cases.
Personally, I don't see life in jail without parole as that much more humane of a punishment than death. I think the death penalty is actually more humane and merciful than locking someone up in solitary confinement for the rest of their lives.