Yes, that's a problem. If he had done something really horrible, surely he would know it.
Maybe he knew all along that he hadn't been very good in his life, but didn't know what had been his worst deed, the one that doomed him to be a Reaper.
I think Charles Frederick aka MeddlingMonk used something like that in his story "The Sprouting of Don Copal". Just like Manny, Domino wasn't told what he had done. He knew he hadn't been a saint in his life, but didn't know what he had done. read the story if you have time, it's very good.