In reply to the first two arguments:
An eye for an eye is a great way to live 'till the whole world goes blind.

The fact is that immoral things happen every day, and this makes us feel justified to do things twice as bad to the "evil" doers. But ethics tells us that capitol punishment and such is just saying "well you killed a guy so we'll kill you; you'll learn you lesson . . . post mortem." So here we're just saying "well you're here, so I'll assume you're a terrorist, strip you and force you to make a pyramid." That's not punishment, that's sadism and the people in charge are putting blame on lower officers.

"Burn them all, the world needs more light."
To kill someone out of convenience is to put ones own life before anyone else's. Killing one whom presumedly deserves it is putting one's actions ahead of their life. The most powerful people are the ones that have the power to kill and doesn't. So we could kill all those people out of convenience, or we could let them live and not needlessly torture them.