Then you should be in favor of Peterson's eventual execution. His would be the exception to what you see as some 'rule'. It would show that Justice CAN be done, sometimes; and not to dismiss the legitimate complaint that some went to their execution innocent of the charges. Not the case, here. And you don't sound glad for it, in that way.
No, you don't decide an issue like this on a case by case basis. Remember, I said I was against the death penalty. Not because there are not guilty people out there that deserve it. But because there are innocent people out there the will be found guilty that don't.