this is a question that will never be solved. Pros: it will make people stop and think before doing the crime. Cons: People who would do something deserving of the death penalty probably are not thinking of the consequences.
There is a need for it. If it is proven without a shadow of doubt, including dna evidence, then the death penalty it is, without stay of execution. It is for the victim's family. And, well, that person, if executed, won't kill again.
This may be a harsh answer to your question. DNA evidence is actually helping get wrongly convicted prisoners off of death row.
So I guess I did answer your question. Yes, we still need it. The punishment should fit the crime....
One other argument has been made. People say if the convicted murderer has gotten religion and is forgiven by God, he should be set free. Well, God does forgive, but that forgiveness doesn't take away consequences of actions.