Who says I refuse to believe it? I'm entertaining the idea - I also entertain the idea he is a miserable hateful person in that same post.
I know, that's why I included the "you probably don't intend to come of like this" part. But to be fair, the majority of your post was expressing sympathety for this person, and leaning against explaining the situation with mental illness.
It strike me as almost a deep structure of the way we think, or how a lot of people think, maybe even subconsciously, that we are so easily inclined to associate (or entertain the idea of) white extremism with mental illness, while per default having no problem what so ever with judging non-white extremism as terrorism, pure evil or whatever.
Just a general observation, I don't mean to single you out or anything.