I would point out that he and the twin sister did bully their little sister quite heavily on occasion. More than just normal sibling bullying/fighting i would say. That would make him an asshole, until he actually is sorry for it.
The question is what your choices are as well - the way Willi talks about Katie, the way you can escalate things with her...
Even when you decide to do a 'good' Willi playthrough, his base actions are to poke fun at people, ignorant of consequences. He actually comes across as emotionally inconsistent - he'll defend Lucas from Kat, then suggest Mon could make more money in porn (even if you decide to be apologetic with her).
So I'm conflicted, either he's perceptive and keeps his thoughts to himself or plays people expertly (like the things he noticed with Donna, the subtle way her gets Katie to give him info about Kat) or he's an oblivious oaf who bulldozes over people's feelings (his souting about Katie in the gym is a good example, the stuff with Mon, the war with Katie etc). Either way he's manipulative, just like Leia and Katie, but he's also very impulsive.
He does have some redeeming qualities, otherwise the circle of friends wouldn't tolerate him and the family wouldn't be that glad to have him back. It'd be "oh look who slunk back with their tail between their legs" the whole time. Clearly he's not an asshole because he's evil, but because he's insensitive. Is there anything to say he's not on the spectrum (I mean heck we all are in some way)?
