But it's going to completely trash his monster self image if he ever finds out!
Slipping into seriousness here, no, I don't think so. Because finding out you're not a bad boy is nothing like finding out you're not a monster. And most people who are afraid they are monsters? Well, I kinda think they don't get off on playing the bad boy.
Then again, I may be generalizing from a small sample. But, still being serious, I will say that a person I loved very much was afraid that he was a monster. And he would not have made a good bad boy at all. Wicked, now, that's another matter. Wicked looked good on him. But not bad.
Bad's way too tame for monsters. No, not tame -- it's playing at something that's too dead serious to play with.
I've been searching for the right metaphor, and all I can come up with is that to somebody who's afraid they're a monster, just being a bad boy would be doing a kind of poorly done, half-hearted drag.