I really like the idea. But I might switch it up so that he looks into the mirror and sees things yet to happen maybe a day to a week in the future. Then have the events actually happen to him. Like in a hotel room and seeing him having hot passionate sex. Then later, it comes true and he happens to glance up in the mirror and see the same scene. Now when he sees the killer look up at him, he KNOWS he will be present at the murder scene. How the hell can he stop it, save her and himself. Maybe he even tries to find the girl to stop it. Or runs into the killer who has no clue who he is, and the guys tries to track him. Given his institutionalization, no policemen are going to buy his story. He has no clue when it will happen only the who and where. Really cool idea. I'd write it up! it would make a good feature.
I dunno, I think it would be creepier if he stuck with the "seeing the past" version. I really like the idea, by the way, and I honestly felt chills just reading your short description of the guy witnessing the murder in the mirror.
I've the same issue with about 60 feature film premises and have a solution for you.I have about 6 other ideas that I'm absolutely positive could be great features. I just have no idea how to go about putting them on paper with life getting in the way of them. I only wish I had a 1/2 hour with Christopher Nolan lol.