Very cool idea, and the first half was really awesome. The protagonist who doesn't know what he's done, but does know that he's done something terrible is an interesting dilemma. In that situation, I'm not sure that I'd want to know what I'd done. The whole point of the erasure procedure is to allow you to have a 2nd chance, making you a new person. If I'd known I'd done something terrible, and my mind had already been erased, I think I'd just go ahead and give that a try. Maybe I'd relapse, but at least I wouldn't seek out my own downfall.
But my interest level dropped substantially in the 2nd half. Suddenly he goes from a concerned citizen frightened of his past, and then when he discovers a bit of his past he says "Killing sure is fun. More please!" Since the only thing that had leaked through up to that point was muscle memory stuff, to have the whole personality and large portions of memory suddenly come through didn't make sense within the context of the story. And then he's able to call up his alternate person's skills on command, when before it had been an adrenaline/muscle memory thing--that was just too much.
What it comes down to was that the first half featured a protagonist who was conflicted, interesting, someone I could try to understand. In the second half the protagonist was just a serial killer caricature.