Right here!
Born in the 80s and a teenager in the 90s, never once did I get 'scarily angry' with anyone or anything - the most was probably chucking a megadrive joypad on the floor, or a rapid on/off of the Playstation when I lost something. But I wouldn't feel the need to take myself off somewhere and hack away at anything, nor would I have any desire to inflict pain or suffering on anyone else for the minor blips. And I had a completely normal teenage development.
My thoughts are... someone born in the 90s and a teen in 00s would likely be different - what with the internet and broadband being the norm. And kids born in the 00s and teens now, would be completely different again - what with 'social media' being the norm; and all the fake personas that go with it - I forget the name of the professor I recently watched, but he was talking about how young people now, are often living a life within social media - as if it were reality; like some sort of Matrix-esq existence!
Of course I am only focusing on the impact 'tech' has on growing up, as I am sure that there are countless other factors that will influence the outcome on someones makeup in life; but "these days" (sounding like an old duffer now) it seems like the interactions between teenagers, primarily takes place virtually via social media - that in itself (imo) really helps to dehumanize people towards each other. Also, I expect that young impressionable minds, become used to being able to do and say things to others, without it impacting them emotionally - in the same way I'm sure some of us, have gobbed off on Xbox chat or your chose PC [ahem master race] comms app; you won't get a punch on the nose, so you talk **** and move on.
Early exposure to political rhetoric is a very good point, as there have been some pretty bile-filled comments from the likes of Trump, and again, a young impressionable mind will probably gravitate towards that. If there is merit to the whole thing about young white males being actively pushed down (so to speak), then they may view the extreme views of some, as a cause worth fighting for - to give their own lives meaning maybe.
But I still think that there must be an element of mental illness to these mass shooters - or we'd have hundreds, maybe thousands more.
Just my 2p.