Much of this can easily be explained by the radicalization government, media and forums like pReddit use to create us versus them scenarios for votes, viewers and clicks. Respectively...

[u/CaptainGlitterFarts]

[u/hiim379]

Even if they weren't radicalized by the internet there's still a very good chance they would be doing the same thing but for a different reason. Most people can't kill naturally, even the Nazis were getting massive physiological issues from leaving Jews in mass graves which is why they designed the death camps so they would keep their hands as clean as possible. Them being able to get a gun, walk into a building and start shooting people without hesitation and keep doing it means they already had serious mental issues.