I never got the "the state shouldn't have the power to kill people" argument against the death penalty. In that case, we shouldn't have a military either, and cops shouldn't be allowed to carry guns.
There is a difference between defense and killing someone in your custody.
Yeah, this. Death row inmates escaping is a scary idea, but one that really doesn't happen. Keeping the worst offenders under lock and key so they can never commit crimes again is something we do well. At that point, protecting society is not an applicable argument for killing them, as it is for soldiers fighting in a war or police confronting a criminal at large.