No need to get clinical about this answer. We have a perfectly good agent noun, 'self-hater', formed from 'self-hatred' (= 'self-hate'):
Hatred of oneself, esp. of one's actual self when contrasted with one's imagined self.
["self-ˈhatred, n.". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/175271 (accessed January 08, 2016).]
As the "esp." highlights, the hatred may be of an actual self contrasted with an imagined self. So, 'self-hater' works well in the OP's example:
He always said he hated mirrors and I had assumed he was a self-hater, so I was shocked when I saw him apparently admiring his face in the glass!
It should be stipulated, however, that what the subject of the example saw in the "glass" might've been something considerably stronger and more admirable than himself.