I'm against death penalty. Even if there were any justification to judge whether someone should die or live... it would be better to let a guilty person live than to kill an innocent one.

That's a 2 - 0 score against death penalty.

PS: Yes, some crimes are so bad that one's emotional reaction is "this guy should die" when hearing the news, but justice shouldn't be based on emotional knee-jerk media-inspired reactions.

Well, the death penalty is barbaric. Period. It's a fundamental violation of human dignity, gives the state way too much power, and ends up being inflicted on mostly poor people, who can't afford sharp lawyers. Lock the guilty up for life, that's punishment enough. But there's a deeper paradox, for you fans of firing squads:

Case 1: Kill 1 person, you're a murderer.
Case 2: Kill 10 people, you're a psycho.
Case 3: Kill 100 people, you're a serial killer demon from hell.
Case 4: Kill 800,000 people by lying us into an illegal, criminal colonial war of aggression in the Middle East

Grant clemency to Case 4, and you can't very well deny it to Cases 1-3, can you? Ultimately, we have to stop being so goddamn violent to each other on this small and very fragile planet. The only thing which deserves to die is the death penalty itself.