Islamic fundamentalists (extrimist =/= fundamentalist) follow quran fundamentally as the absolute truth from god, so to say that he wasn't corrupted by islam is pretty thick. Just because most muslims don't believe in killing gays (alltho many muslim majority countries have a punishment or even the death penalty for homosexual acts) doesn't mean its not islamic. That's like saying a neo nazism isn't influenced by nationalism but instead fundamental nationalism, makes very little sense.

I keep saying it time and time again... just because a person is not evil (like most people aren't), it doesn't mean the ideology he supports cannot be inherently abhorrent. So the whole "most muslims are good, so this guy wasn't a true muslim" doesn't really work.

Also I did not bring IS, into this discussion at all, so I don't understand why you brought that up in the context of my post? IS doesn't have the monopoly on islamic extrimism.
Wether he did the attack in the name of IS, Boko Haram, Al qaeda or just allah in general is mostly irrelevant to me.