Apparently this wasn't "terror related". Now, aside from the fact that the dude wandered around town with a shotgun terrifying people - and killing them, including a young child who I'm sure wasn't experiencing joy at the time - it seems pretty clear he's been radicalised by a gaggle of absolute arseholes.
He appears to have described himself as "blackpilled". If you're not aware, blackpill is the stupid cousin of redpill - and given how stupid redpill already is, that's going some. You might have never heard of either (lucky you), so...
"Redpill" refers to someone who is absolutely convinced that they see the world as it truly is, rather than the lies fed to you by woke left-wing media, celebrities, and politicians. They think there's a global conspiracy to marginalise what they see as maleness, and eradicate male rights. Anything that promotes nicer, kinder men or condemns thuggishness and the culture of possession and sexual assault of women, is part of this "woke" agenda. Homosexuality and the entire concept of transgender is part of it too.
The name comes from The Matrix, where Morpheus offers Neo the chance to take the blue pill and go back into the comforting lie of the Matrix, or the red pill to see the world as it truly is. Of course there's a massive irony here in that the film was created and directed by two trans-women under their previous identities as brothers (and the red pill itself may symbolise oestrogen tablets, though it's nowhere near that size); an ideology that sees trans identities as anathema is named after a concept created by trans-women.
"Blackpill" goes further and essentially believers think that the concept of marginalising men only truly applies to men who are not traditional "alpha" males; they think that "real" women are more attracted to "real" men who keep traditional male values of being unpleasant and unkind to other people (especially women) and that although women don't acknowledge it openly they do in fact prefer to be verbally and physically abused and raped by partners who do this to multiple women simultaneously.
While redpillers are often incels ("involuntary celibates"), blackpillers are exclusively so, and they believe they cannot have sex with women because they are not the kind of men that women truly like (bastards), and they are destined to die single and as virgins - blaming it on women (who are sluts and liars) and weak men who permit the advance of female emancipation - and as a result, the belief is pretty close to existential nihilism. It's not much of a surprise when one of them takes themselves out, though they mainly live on dedicated circlejerk internet forums bitching about sluts and soyboys while talking a big game about suicide and murder for 18 hours a day.
It's also even more moronic because there isn't even a black pill in the film.
I don't know how that isn't radicalisation, and I don't know how someone making YouTube videos about his murder agenda while going out to kill innocent members of the public in the name of his agenda isn't terrorism.