Hi There Anon – Admin Liz here. I think the reaction comes from a feeling of betrayal. At least it does for me. While it’s always been clear that he’s been out for himself, he still had people who he seemed to care about (or at the very least trust and respect). He betrayed all of those people in this season.
Also, I’ve never considered him a pawn – he’s always had a lot of strength and confidence, especially in his power as person, half-Royal, and half-Zauberbiest. In this year’s finale specifically, he seemed to lose all dignity he previously had and just let Boneparte play him, and the people he had worked with the last four years (more with Nick, Hank, Wu, Meisner, and Adalind).
I, personally, have no real issue with him going full dark side. I don’t even really have a problem with him betraying Team Grimm, because you’re right, that’s not out of character for him at all. What I do have a problem with is that he suddenly lost all of his ferocity, and I kept waiting for him to at least bear his teeth, something, anything, to try to take some of the power that Black Claw promised him, but he didn’t. He was just as much of a hostage as Adalind, Diana, and Kelly, but unlike them, he was just rolling with it. He had no problem just rolling over for Boneparte and everyone else. There was none of that Sean Renard swagger that he’d always had.
So… yeah I guess that’s what I’m personally most upset about.