@Ilay I think the mirror is making him give into his inner desires. Horus has a part of him that wants Seth carnally, but he supresses it because he knows it's wrong . The mirror amplifies that feeling and throws all gone his sense and reasoning, all the control. Horus is not a bad guy. How humans have those thoughts, but we know it's wrong and so they'd never act on it. It's just that the mirror is making him completely encompassed by this thought and emotion, so his mind is gone right now. Because you see his eyes the same eyes as the mirror like he is controlled by it.