The idea that he must have been radicalized by some external force also ignores a very long history of humans developing feelings of detachment and rejection from what they believe is the society around them . . . a state of mind that, in some, can become violent entirely on its own.
I do think that some people become radicalized by organized systems (including online material) that foment these feelings in individuals - and the more the individuals become attached to the organization, the more pliable they are to carry out acts. But that certainly isn't essential - just because someone decides they're going to kill people while brandishing and ISIS flag doesn't necessarily mean they were radicalized by some online material or local agent.