He’d be a shitty person whether he wrote the Turner Diaries or not. What the OP is saying is that someone’s writing is not proof that they are an immoral person. Tools of analysis are not revealers of truth. Extreme examples shouldn’t make the rule.

I would say being the author of the Turner Diaries would make you an immoral person even if you never did anything else bad in your life though, the fiction can reflect the author's morality and there it did.