EDIT: I had to go check facebook myself since I'm not going to automatically trust that some imgur posts are actually the same person. Given the
long history of misidentification of shooters as Democrats, you'd be foolish to take some imgur photodump claiming political alliegence for a killer at face value without at least checking it out yourself.
Well, he checks out as the same person, but I doubt he was too much of a "Bernie Supporter" so much as against Hillary. Basically a right-wing anarchist who saw Bernie vs Trump as the best middle finger to the establishment. The imgur image dump was pretty much cherrypicked if you read the guy's facebook page. There are lots of anti-left, anti-liberal, pro-nazi stuff on there too.
What I know about him: dope-smoking, comic book geek, into heavy metal, who's also an anti-authoritarian right-nationalist. People were dumbfounded that anyone on the right could like Bernie better than Trump, but that's a black and white worldview. People are complex ... you get someone with these interests and they can still be the biggest racist fuck, but be tilted more politically towards Bernie than Trump.
(weirdly, the killer has their gender set to female on facebook. it says send her a friend request to see what she shares with her friends)