I am in favor of the death penalty but the circumstances have to be appropriate. Sometimes biological automatons are just flawed without any hope of rehabilitation.
I'll give you two scenarios which I believe are important to this discussion which express my position well.
1: Serial Killer
Well we know that these people tend not to stop what they are doing. I am in favor of terminating these flawed humans to prevent the rest of society from their burden.
2. Husband comes home and finds his wife sleeping with the neighbor. He flips out and kills one or both of them.
This doesn't seem to be a future risk to society, it seems more of an emotionally fractured person caught in a moment of time. Since the risk to society as I see it is negligible, if not zero, I would be in favor of a short sentence (if any sentence at all, but that's another discussion).