I really liked this story for the first half, and then it sort of lost me a bit. Not a huge amount - I think it still is a good story - but I just felt that it ended up short of what it could have been.
The problem for me was as follows: it started up by setting up an intriguing psychological drama. The narration by the erased person was intriguing; the possibility of having an unknown past, and having to live with the questions, was great. And the tension between living in the present and the shadows of the past was also very well done.
But then, the story made several missteps. First, the narrator displays a lot of concern with the physical skills associated with his past, but seems to click back into being a murderer with hardly any qualms at all. This is not the story of a man fighting against dark urges, it's the story of man, discovering his dark urges, going "huh, that's where that was", and easily adapting to it. That makes it pretty ineffectual as horror, since I can't emphasize with him anymore. In order for this sort of horror to be effective, I need to either be able to place myself in the position of the victim or of the perpetrator. Without empathy, all we are left with is the fact that some people do horrible things to other people. That's not horror, that's reality. I can learn as much from picking up any newspaper.
The second misstep in my opinion is the fact that the voice in the narrator's head goes from being an abstraction early on to being a full-fledged second personality, one that can hold discussions with the narrator, take over his body on request, and basically act as a fun little friend. This totally undermined any realism attached to the notion of erasure. I'm not arguing about whether the story is plausible. I'm perfectly happy to buy into the notion of mind erasures, and the fact that they can be imperfect, and that stuff can "leak" through. So far, so good. But the fact that the "leaking" can take the form of a fully functional personality with its own memory and independence is just outright silly. Again, the story started from one premise - struggling with a hidden memory, and ended up somewhere totally different. And worse, this seemed totally pointless. It felt like either the author was not giving me enough credit ("the reader will not understand that the old personality is coming through if it's not made into a character"), or that he was lazy.
And mostly, these flaws were a shame because they obscured much that was good about this story. The fact that the erasure was faulty was clear from the get-go, but the fact that the new personality combined with the old to make a worse person was very cool. It's a shame it was overshadowed by the silly multiple-personality thing. If the narrator wasn't as quick and happy to become a psycopath, and the depiction of the dual personalities coming together played subtly instead of as a Saturday morning cartoon, I would have really loved this story. Now I liked it, but I can't think about it without wishing it was simply better.