In your worldview.
You see, while the moral of murder being wrongful is (virtually) universal, the punishment for such act is not. However, related concepts are.
You see, the thing is, eye for an eye doesn't really exist. It's more of a 'they did something bad, let's do something bad to them back' situation than anything. That is not justice, that is revenge. Justice is about setting things right and keeping order. Revenge is about satisfying an animal instict that is on the most base level of human existence. There are people who are perfectly okay with that, the people who want to 'satisfy their natural yearnings'. Oh, goodness, aren't people like that often criminals?
The people who have said they believe in eye for an eye, do not believe in it, as the concept truly stands. They may believe in relatively harsh retrobution, and they can call that eye for an eye if they like, but it is not. Eye for an eye involves directly returning a wrong onto the head of whomever commited it. Civilized people don't do that, we started to figure this out in something hundred B.C.E. and the third grade. Unfortunately, not everyone gets it. Those are the people, who generate disharmony. Those are-often-the criminals, the war mongers, the hateful wretches who fill the wonderful media with horrid images to give the good people something to be apalled toward. But again, they are people, they are the same as we are, they just have a different view of the world, one which is (because it cannot exist peacefully alongside that of most people) wrong.
Consider the law. If you see a man on the street rape a woman, in full view, and then shoot her in the head, and you return the favor by raping him, and shooting him in the head, will you go to jail? Yes, you have commited a crime, no less heinous because you did it to someone who had just done the same thing. This is where an odd incongruency emerges between the death penalty, and American society (and more importantly, civilized society, of which America is only a part).
Even if the majority of the people in the world wanted the death penalty (debatable) that does not mean it should stand. It deprives people of what is widley held to be one of the basic human rights;
Life, Liberty, and the Pursuit of Happiness (property)
It's the first one! This phrase was originated by John Locke, and changed from property to happiness by Thomas Jefferson and his posse. However, it is fairly universally held, that these are the things which all human beings, regardless of race, religion, social standing (criminals alike) or moral character, are entitled. If you choose to disagree with this, you are setting yourselves at odds with the body of the civilized world.
(edit)Something obvious I just realized. Read your post. Anyone who willingly kills someone deserves it? What then do we deserve for the killing of criminals who 'deserve it'? Shall we all be killed as well?