I think that a human life, no matter how depraved, is at least worthy of mere existence. If a marginal fraction of society's resources need to go towards his continued existence, I think it's a cost that's more than justified.

Also, I think the evidence suggests a different motive for the death penalty. I think it's a facade that the death penalty is in place to protect society. There is no evidence that I have seen that the death penalty acts as a deterrent, or that there are lower rates of murder in states that have imposed a death penalty.

Furthermore, we don't kill people for protection. We kill people so we feel that we have the right to pass moral judgment on them. For instance, he who is declared mentally insane, with no hope of rehabilitation, is allowed to live, albeit in a mental facility. He has no hope, as far as we know, to be rehabilitated. He is more of a danger to society on the outside than a mentally competent person who let his emotions get the better of him. Yet he's kept alive.