Why so scary?: Unstoppable evil
After being described as "pure evil" by your psychiatrist, it's likely that you're going to be the frightening type. Add a mask to the mix and the resulting panicked reaction is inevitable!
Following 'Halloween', killers in masks became very much en vogue and few slasher movies these days would be without some sort of mask making an appearance.
The reason masks are so scary is that they dehumanise the killer, making them an unemotional and detached murdering machine. This then leaves the viewer focussing all their attention on the terror of the victim.