Something that interests me on this topic is that those who are in favour of the death penalty are more than often also the same people who want to see captured terrorists suffer, rather than receive immediate (or, 6 months down the line) death in a relatively humane way.
Even without the "easy get out clause" point for deluded fanatics and religious murderers/terrorists, state-endorsed killing is still killing IMO.
It just makes no sense in some parts of America anyway, some murders are punishable by death but others are not?