I couldn't really care less about murderers, etc. being put to death, but my issue with the death penalty is it's inevitable that innocent people are going to be sentenced to it, and it offers no real benefit other than some sort of karmic gratification. Anyone committing a serious crime like a murder, is either of them mindset that they're going to get away with it, or has given no consideration to the consequences, so the severity of the punishment is going to have no bearing on it.
I also have very little faith in the US law enforcement and judicial system, and its still far from the worst in the world.
Life imprisonment without parole is a better alternative in my view, it's just as effective as removing that individual from society; and in the cases of wrongful conviction, allows the mistake to be rectified to an extent. It's also probably a crueler punishment, if you're into that aspect of the death penalty.