I can’t and won’t speak to this Belleville case specifically. Speaking generally, and without intending any suggestion to the facts of the Belleville shooter:
In the radicalization context, society is now facing the reality of a generation and a half raised as internet natives, with ready and unsupervised access to awful online spaces that have matured and evolved for years in the Darwinian world of ‘awful ideas’. We have both kids and also now adults raised on a diet of the most malicious disinformation by predatory actors and grifters, inculcating foul beliefs that metastasized well beyond the initial audience. We have rampant and sometimes deliberate desensitization to horrendous violence. We have constant ‘othering’ and dehumanizations along all manner of social, gender, economic, religious, political, and cultural lines. All the people who used to be the school loner or misfit now find and reinforce each other, build their own communities with radically different values, and in some cases celebrate and glorify those who become literally murderous.
Most of the kids are still ok, but a lot aren’t, too many parents are abandoning too many kids to hours in their rooms on the internet, and they’re getting badly fucked up. A relatively rare few violently radicalize, but even that relatively rare few have become a huge problem staring educational, social, and even law enforcement and intelligence resources.