What always bothers me about this sort of argument is that those who are against the death penalty almost immediately try to seize the moral high ground with talk about 'descending to their level', and that those who are in favor of the death penalty are inclined to let them - the only moral argument you hear is that it's morally neutral to execute a criminal.
To pull out some Kant, let's say that it's actually those who would choose not to execute the murderer that are sinking to his level. You're both utterly failing to give someone what they deserve, that being the very definition of justice. The murderer's victim deserved not to be killed, and the murderer deserves to die (in line with the categorical imperative). There's a case to be made that someone who wouldn't execute a murderer is just as culpable as he is.
Come on people, don't give away the moral high ground.
Also, Locke's social contract doesn't really say anything one way or the other on the death penalty. The whole point of the thing is that you're in a -contract-. When you violate it, the state can punish you by restricting your life, liberty, and property.
Edit: I note that alpha's post was more sophisticated than I at first thought, so I'm going to expand on that last bit.
What I object to is the idea that society somehow legitimizes murder by engaging in execution. The reasoning for this strikes me as somewhat questionable. You say that "In taking a life for a life, the society is admitting that taking a life can be okay, which removes any sort of authority the society had over that individual." Would you also agree that society can't choose to imprison someone whose only crime is to lock innocent people in cells? As well, isn't the very act of imprisoning a murderer an admission that imprisonment is okay, which removes society's authority to punish those who imprison?
It also seems rather senseless to maintain that society isn't approving of an action when it imprisons a person, removing them from society for some period of time, but is de facto approving it when they remove them from society permanently.
What you're missing is that these arguments are simply forceless against anyone who believes that an action can be moral or immoral depending on the moral standing of the object of that action. Society doesn't need to operate by categoricals such as "killing is wrong", and it makes perfect sense to phrase our categoricals in different ways, such as "intentionally killing one who has not committed murder is wrong", "killing one who has committed murder is okay", and then defining murder recursively as intentionally killing one who has not committed murder, with the understanding that no one is born having committed murder. The first intentional killing qualifies as murder, but the execution of this person does not.
Such a view doesn't make for as nice of a sound bite, but it's perfectly consistent with everything but some strains of utilitarianism.
Sorry for the length, but I'm almost done.
This is actually the whole problem with trying to define morality via categoricals (such as thou shalt not kill) - which categoricals do you pick? Sure, if you've chosen "killing is wrong", then you can say that the state sinks to the level of the murderer. If you've chosen "intentional killing of those who have not commited murder is wrong", then there's no problem. And there's no real way to tell which one is better.
