I think this boy is deeply misguided and I fear he might radicalize further. However, I do take exception to saying that posting 'X is deadly' constitutes "bullying an entire religion". I'll requote what /u/4991123 posted above:

https://i.pinimg.com/474x/05/40/6e/05406e320ef5c39b7e5180aec1c6fdd3--rowan-favorite-quotes.jpg

Back in my edgy atheist teen phase, had non-anonymous shitposting on social media been normalized then, I might have posted something like "Christianity/Islam/Judaism kills" too. (This was after I lost my Catholic faith by actually reading the Bible and reading the horrid and backward stuff that was in there besides the "Jesus being nice" stuff that we did cover in school). Growing up you realize that this isn't the best way to get people to listen to your arguments, but it is pretty much what I honestly thought back then.

Of course religion isn't a unique source of evil in this world (and can sometimes be a source of good), and repressive anti-religious policies are often just as bad if not worse. But I always did feel a discrepancy between how people react to criticism of an ideology and criticism of a religion, because ultimately religion is a form of ideology. But because religion has a veneer of sacredness people take it much more personally and it's interwoven much more into their identity. In some countries, like Canada, some religious people even get exemptions from certain laws. Add to the fact that he's singling out one religion (and it can be debated that Islam isn't inherently uniquely terrible compared to other Abrahamic religion), associated to an ethnic minority that often faces racism, and it complicates matters further.

Having said that; doing a thought experiment, if he had said "Catholicism is deadly" instead of Islam, and some believing Catholic kids (as far as those even still exist in Flanders) had taken it personally and threatened him with physical violence, would people then be saying "sucks for you, but you shouldn't have bullied an entire religion", or would people be calling the Catholic kids a bunch of easily offended hotheads ? (Note, I don't want to demonize the other kids either. I can understand that other teenagers feel threatened and offended to the point of becoming aggressive, but it understandable doesn't make it okay either). And what if someone posted "the far-right is deadly" in a class with VB youths or "communism is deadly" in a class with PVDA youths? Those kids would probably get butthurt too, but does that constitute bullying or just criticism of an ideology?

That doesn't mean that this particular kid isn't on a misguided path himself btw, esp. if his other posts consist of him spouting your typical VB talking points. I just think that it's "problematic" to say that criticizing a religion by pointing out the deaths and murders made in the name of that religion constitutes "bullying an entire religion".