Well it has already been posited that systemic errors are inevitable, and that the death penalty logically is irreversible. Also there are the aforementioned extra legal motivations to affect the system, whether those effects are subtle or just serve as a brutal reminder of what happens if you attempt to rebel against the established order.
Fine, so, the death penalty is not always wrong, but the system which governs it isn't perfect either. That being the case the final verdict of whether to have such as system or not has to be a resounding 'no'.