was it seriously the terrorist who caused 9/11?

No, so yes. Conspiracies are invented to provide structure to a random and therefore terrifying universe. The idea that a dozen or so religious lunatics could cause so much destruction so easily, could change the very course of human history, is beyond horrible and difficult for many puny little human brains to grasp. So, a conspiracy is applied to make it seem like there's a plan, a reason behind it. But there isn't. It's just hatred and evil and nothing else.