If a state could prove conclusively that a person was guilty of a capital crime, I would be in favor of the death penalty. As it stands, many people are put to death that aren't guilty of the crime they've been convicted. That's intolerable to me. I'd rather some guilty people go free rather than punish an innocent.
Granted, this is fairly idealistic and it's probably impossible since humans are flawed. The most pragmatic approach is to not execute the convicted, but the imprison them. Then, if new evidence surfaces, a new trial can be had.
For more reading, visit
The Innocence Project.