This is all IMO, so feel free to shoot it down, as is your right, priveledge and probably gleeful responsibility
it seems to me that he didn't think that his killing of anyone was wrong, even seemed like he believed he had a moral imperative to do it
he habitually killed, it was part of his life, wheras the death penalty is regularly exercised to punish the single worst piece of behaviour in a persons life this was punishing a long history of a behaviour, an unremorseful, seemingly proud history of massacres, injustice, and bloody cruelty
and further, the raising of progeny to follow in those same blood marked footsteps
he was executed partially so that he would not be able to be freed and made a hero were the iraqi civil war to go "badly" as though a war could ever go well, also partially because it is a morale boost for a certain sector of america, generally those most responsible for supporting this war, the republican base <not saying there aren't dems, libretarians and out right marxist communists in the armed forces, just saying that it is on the whole more of a "red state">
need more troops, boost morale
sucks, but it's the world we live in