That is a long and complicated question…and I’m not sure I would call him evil, necessary. It’s just that his loyalties have changed drastically since season 4. Not always for the better, either.
And like a lot of fans, I’ve never been satisfied with the reasons for Sean’s seemingly sudden change in season 5. I can headcanon some stuff together for it, but IMO something as pivotal as a major character’s motivation behind a complete 180 on loyalties shouldn’t be left up to headcanon, or at least not entirely so.
My best guess is that when Diana was taken by the royals and that helicopter went down, Sean honestly didn’t know at first whether she’d even survived…and then he didn’t know where she was for several months. I think that affected him far more than we initially realized as an audience, because when I look back it was right after that when Sean started withdrawing from the team quite a bit.
Enter Black Claw, promising power and influence and protection…and then giving him his daughter back, alive and well. It was only after they gave Diana to him that Sean really got on board with their plans and seemed to drink the Kool-Aid.
So I think it was a combination of things that made Sean go bad, not least of which was a lot of unspoken resentment, fatherly grief, and Team Grimm’s tendency to do whatever they think will solve the problem of the moment without always considering the long-term consequences or collateral damage.
That being said, I do think you should catch up as long as long as you’re not triggered or deeply squicked out by Nadalind (there is a lot of it in seasons 5-6). Their scenes are more numerous than I would like, but they are mostly brief. And the show improved drastically in season 5 and has been as good as ever thus far in season 6.

