I absolutely love it! Of course, I have a thing about mirrors. As a small (and weird and creepy) child I used to stare into a mirror and look at my reflection, then start to smile. I would imagine that I wasn't smiling, it was the mirror image that was making me smile, and that it was taking over. I also used to look down the infinite reflections of a mirror reflecting a mirror and imagine that one of those reflections, maybe 10 or 12 mirrors down the line, wasn't a reflection but a different world, and if I looked long enough I would see something happen that would give it away. Scared me to death and I loved it. Again, weird kid with overactive imagination! But I'm fine now....
Anyway, yeah, mirrors can be creepy, doubly so if you start playing with them. You could work lots of things into this, like a bit of lag, where he sees himself, then moves, but it takes the reflection a moment to move with him. It could also be pretty creepy if he sees the past, but still sees himself in the mirror, with the other people/events taking place in the background.
Love the idea. Some constructive thoughts: I don't think the visions should be explained. Not that you were necessarily planning on it, but I like the idea that it never makes sense to the character; by the time he's let out of the asylum, he doesn't even try, just avoids the subject when he can. I like the killer taunting him through mirrors; knowing that he can't prove anything (being terrified of his power, he wouldn't spend much time looking closely at the events in the mirror).
So maybe the killer gets out, and reconnects with Mirror Guy. He drags him to places where he committed crimes, and forces him in situations to look into the mirror. He makes it clear as he's committing the crimes that he's setting Mirror Guy up for them (then he can't go to the police, because he would know way too many details about the crimes). In Real Time, he's super nice, helping his friend who has never managed to adjust to the outside world, setting him up with dates (who he will later kill), helping him get jobs (where he will take advantage of to rob places, or sabatoge companies, etc), making him meet new friends (forcing overwhelmingly socially awkward situations, making him do things he wouldn't, like run into a bathroom with lots of mirrors).
I love the idea, and can't wait to see where you go with it! And as an aside, if you haven't, there's an episode of Tales from the Darkside you need to see:
http://www.imdb.com/title/tt0716949/ Not much to do with mirrors, but the ending scene is one of the creepiest and saddest scenes ever (I love it!)