Besides,
Ren, surely you've noticed that every time some disease or other gets a somewhat positive media coverage, and a crop of "celebrity victims" many people are– for a little while– convinced they see it in themselves, their family members, historical figures, characters in Shakespeare... etc., etc. In fact, there's probably about fifty other things you could "diagnose" Melkor with just as easily.
Was he in fact, crazy? Well, I certainly wouldn't call him a paragon of mental balance, would you? But I think it's just more valid to leave it at at "generally, amorphously crazy" rather than trying to pin a medical diagnosis on a fictional character, especially one who isn't even a human being.
