Mere factual innocence is no reason not to carry out a death sentence properly reached.
I wrote a million words in the first year, and I could never have done that outside of prison.
I can tell you this on a stack of Bibles: prisons are archaic, brutal, unregenerative, overcrowded hell holes where the inmates are treated like animals with absolutely not one humane thought given to what they are going to do once they are released. You're an animal in a cage and you're treated like one.
Whatever you think of de Sade, he was a complex figure and we should not look for easy answers with him. He was, strangely perhaps, against the death penalty, and he was never put in prison for murders or anything like that.