Perhaps he is a Horcrux transfer (as long speculated) but a failed one; introspecting about how different he is from his memories of 'himself', he would realize 'he' hadn't survived and all that was left was a weird mishmash of Monroe's personality and Voldemort's memories, and this was entirely worthless as immortality.

If Monroe was a hero, then Monroe's personality really doesn't fit with some of Quirrel's actions. But there are times when it seems like Quirrel is developing some affection for Harry, for instance the Christmas present.

I have long been wondering if there is a possibility of Harry redeeming Quirrelmort, and if there is actually any part of Quirrel still in there, it makes this idea a lot more plausible. I had thought that after the redemption, Harry would deduce that Quirrel is Voldemort. But I think there is not enough time left in the story for this to play out.