Even if it does deter, most Death Penalty opponents come from a human rights and moral direction, so any kind of utilitarian argument isn't going to matter to them. It just seems logical to me that if the cost of an action is higher, that behavior will be more deterred. And while AJ's data says otherwise, I agree with Lucrative's idea that states with the death penalty may have more problems with homicide, so they take extreme measures, and it would be interesting to see how homicide rates would change if say, Texas revoked the death penalty.
Statistics is just one of those things that I do not think is THAT well equipped to deal with complex sociological issues, or any chaotic/complex system for that matter. It can be helpful, but they're also prone to misinterpretation and cherry picking from both sides of the argument.