If we can be certain, then I could be for it. But because we can not always be certain, I can not be for it, even in the obvious cases of guilt. We are not always smart enough to discern those obvious cases. If we are not able to discern every obvious case, then we leave ourselves open to committing a mistake, putting an innocent person to death. I am not willing to take that risk.
To answer your question more directly, no matter how much the death penalty may seem deserved or how much satisfaction might be derived from putting a murderer to death, that justice or satisfaction is not worth the risk of one innoncent person being put to death. So, in my view, society gives up on execution of even the most guilty in the name of never putting an innocent person to death.
The only other way I see it is if someone wants to argue that our system of justice is infallable or that they think it is okay if innoncent people end up being put to death in the name of condeming true murderers.
You equate Justice with revenge. Those are not the same.