Let's go ahead and call this person Bingo-chan, as advancedclass so aptly named them.
I think they were always headed in this direction. They've been embedded in the tumblr echo chamber and a twitter sphere that just reinforces the tumblr echo chamber for a really long time, and that has only compounded things. They had a "queerer than thou" attitude even before I knew them, and they've always justified a lot of toxic behavior as "self care." They also avidly play the Oppression Olympics game. Over time that's magnified their sense of entitlement and driven them to become more and more special and consequently more oppressed because they are so very, very special.
A short example: one of their roommates at the time legitimately had the problems that Bingo-chan liked to pretend they had: mental illness, poverty, and oppression (this roommate is a POC). Becuase she actually had the problems Bingo-chan's identity was based on, Bingo-chan treated her horribly and belittled her problems because they were threatened. Most of Bingo-chan's expenses were covered by their other partner* and their parents. I don't care if someone is supported financially by their partner or their parents, because it's none of my business, but claiming poverty for internet cred while getting that kind of support is pretty fucked up.
*By dating me as well as this other person, Bingo-chan bumped themselves up the queerer than thou ladder one more rung by having a polyamorous relationship, which was topical at the time. I am not polyamorous, but I was willing to look past that because of the situation detailed below.
I had a couple months prior left a two year relationship. My ex from that relationship (let's call them G) was jealous and manipulative while we were dating, and it only got worse after we broke up. A lot of our friends recognized that what G was doing was wrong, but wouldn't say anything about it because he was valuable socially and/or because they didn't want to "pick sides." I felt very lost and alone, and G's behavior was taking a toll on me mentally.
I needed someone to back me up, and Bingo-chan made themself available. They and their other partner stood up for me publicly, which is what drew me to them. This was someone who felt strongly on my behalf and actually wanted to help (although they never really did - in fact, they made it worse). They were involved in the social circle I was a part of, but they were just far enough removed geographically and socially that I could get away from G. It was a safe place, even if it was a shitty one.