Saying that youtube/conspriancy theories caused his to shoot people is the same as saying that metal music or video games caused this.
A mentaly ill person is a mentally ill person: it doesn't matter what triggers him because he is mentally ill and then they'll point their finger on the thing he read/listened instead of focusing on the fact that he was mentally ill.
No sane person will shoot up a school because of right-wing radicalization or video games.