that was his religion, that was what he identified as, that he wasn't as pious enough in the eyes of some or that he had an interest in what we say is an extreme interpretation of it doesn't stop him from being a muslim any more than claiming an IRA member who hasn't been to mass or confession for a few years and has just blown up a load of innocent women and children is supposedly not a catholic
he believed in Islam, he searched jihadi websites prior to the attack - it was an extremist Islamic ideology that motivated him
how do you know he believed in islam ?
but anyway explain how he was a follower of islam, what actions of his define him as a muslim.