Before you posit that Elliot Rodger must have been mentally ill because stable people don’t do what he did, did you say that about the 9/11 hijackers or the 7/7 bombers or the Beslan school hostage takers or assailants of Mumbai?
No, because they were terrorists?
Well why were they terrorists? Because they had a true belief - one that was possibly written down - an agenda, made a concrete plan, and then carried it out?
What, exactly, is the difference?