Does someone become

"self-radicalised" and, if so, how?
I am not sure about "self-radicalisation". I think it is much more complex than that. I doubt anyone sets out with the conscious aim of becoming a radical. Individuals have to be influenced by other things.
What happened in Canada yesterday was appalling - and yes, of course it could happen anywhere. I even heard someone say, "I wish he'd done it to our mob and succeeded." They probably weren't being serious but it made me, quite literally, shiver.
People say that sort of thing without meaning it at all. They also say things like "I could kill him" and "I could strangle her" when they are frustrated or annoyed with someone. It is not nice or polite but it seen as a way of "letting off steam".
The young Australian in the IS video sprouting hate and death is something entirely different. Is he sane? I doubt it. He has certainly been "brain-washed". His thinking has, somehow, been successfully changed. That is what worries me. How was that done?
Do we have to take some responsibility as a society for that? Do we actually isolate people - or allow them to become isolated?
I work from home. It would be easy to become isolated. I could go all day without seeing anyone but the Senior Cat. That almost never happens though. We might have neighbours come in or, more often than not, I have to visit the library or the Post Office or go to a meeting. There might be shopping to do. I see people I know - or people who know me even when I don't know them. We say hello. It isn't like visiting friends or doing something in a group but it is human contact of a sort. And then I belong to a knitting guild. Knitting is a hobby. I enjoy it.
But I also "lead" two knitting groups and teach knitting to a small group of teens with disabilities. It may sound like a lot but it really means a social activity about once a week. The first two groups are also what I consider to be "support" groups. There are people who attend both of them who need those groups for reasons quite unrelated to knitting. They need the company. They need support.
I wonder if some people have that sort of support. I suspect they don't. They are lonely.
The teens recognised that long ago. All but one of them are now in the last year of secondary school. The other has one year more to go. Will they go on meeting? It will be more difficult but they have been discussing ways of doing it - and I think the bonds are so strong that their friendships will be life-long. Social media helps even now.
A big city, where you are surrounded by people, can be the loneliest place in the world if you don't know anyone. Interaction via an electronic screen is not the same thing. I really enjoy my interactions with many people via social media. I have not met most of them and I know I probably never will. That doesn't matter we can still enjoy a form of friendship and company. But I also know that I need, for my own mental health, to see people in the flesh from time to time.  It's a different sort of relationship.
Social media can however make it easy for us to believe in friendships which don't really exist. If you are young and lonely and someone "friends" you on social media and then starts to tell you in a persuasive fashion that you are "important" and need to do something important then you might well come to believe it.
I know not everyone will agree but I don't really believe people become radicals in isolation. I think they may become radicals because they are isolated.