Death penalty doesn't work plain and simple. It's not an deterrent for violent crime, it kills innocent people and is even more expensive than life in prison due to extensive appeals process which is needed. Unless you want to do like China, and execute them hours after the "trial". Death penalty is not about justice, it's about revenge, and I don't want a legal system based on the idea on revenge.
Even so, death penalty is one of the most cruel form of murder known to mankind. To find an equivalent when it comes to "civilian" murderers, you'd have to find someone who tells his victim that he's going to kill her and when he's going to do it and then keeps her confined until that day, at times giving them slim hopes of escaping death and/or postponing the date of execution. Frankly, such monsters are rare in society, but for some reason it is accepted that the state can act in such a manner.