One in a million death row question, but I am serious about the answer.

I've always wondered this: I'm anti death penalty for many reasons, but one of the main ones is flaws in the system, ie. an innocent man being put to death. What if such a man is on death row, and it's time for his execution. Now I know there's probably many many precautions taken, but let's say in his desperation, he manages to kill a guard. He's subdued, but then, new evidence is found and he is exonerated. I guess the presumed method is the governor calling that last minute phone they have, but I don't know if that's an urban myth. Regardless, right before he's put to death, he kills the guard, and then it's found out that he's completely exonerated of the crimes that placed him on death row.

My question: Is he guilty in the death of the guard, or was it self defense?

Again, I know it's a very contrived situation, but I am generally curious about the legal ramifications.