I think someone else brought this up too, but basically, even for the most heinous of crimes, death is hardly a fitting punishment - i think making that criminal's life as rubbish (and as long) as possible is much more fitting. And ironically, taking the US model as an example - having the death penalty is more expensive than just locking someone away forever.
The prison system generally should be founded on a principle of reform rather than mindless abuse and punishment, though. In the long-term, rehabilitating someone is far more cost-effective and helpful to a society than turning them into vengeful monsters who hate the system even more after they are released.
Death penalty....it's definitely not a deterrent in a country like the US. But I feel that in China, the deterrent effect DOES still exist to some extent. It's damned safe here in China,with violent crime and street crime at very low rates, even in the darkest corners of the biggest cities. Still though, I have no doubts that the effect will wear off more and more over time. Even now you can see it doesn't have the same potency as it used to.