The thing is, in isolation, yes, in a sense I agree they deserve to die.
But nothing happens in isolation. Our justice systems are highly fallible, and mistakes are made. There is no excuse for even one person to die who was innocent, and this happens alarmingly often. More people get hurt when people are put to death, what's the point then, aside from raw vengeance? The death penalty does not work as a deterrent, the only thing that's going to make a difference in a social respect, is dealing with poverty and a lack of opportunity and education.
The couple of violent loons we have, should be securely locked away. All the arguments for the death penalty are highly connected the state of the justice system, and a justice system that is working, will be able to keep people incarcerated, without the need to kill them. If this is not possible, it's certainly not a system I'd trust with people's lives.