J is kind of an asshole and I think we all realize that especially after what she did in Episode 8. Still think she's being hated on a bit too hard though.
She knowingly sided with Cyn, despite knowing what she was responsible for, and that's obviously not a good thing. However, do keep in mind that all of the affected Elliot Manor drones are just traumatized children at the end of the day. Cyn put all of them through immeasurable horrors. Even without Cyn's influence, the drones were treated horribly by the humans (other than Tessa).
And based off the scratched-off armbands that we saw N, V, and J have that also match Cyn's armband, there's strong implications that all three of them are also "zombie drones" much like Cyn is herself. We don't know what they were subjected to before being tossed into the dump and later being found by Tessa.
As J and V are fighting, J makes an interesting comment that sheds some light on her motivations. "...It tricked me, too." Obviously, that was never elaborated on, but it strongly implies that she had made some sort of a deal with Cyn for an unknown reason.
We got confirmation that V also had made a deal with Cyn, which was basically that she'd do whatever Cyn wanted as long as she wouldn't hurt N (and then would leave them both alone). Don't know if N also made a deal, but he's likely there as a part of V's deal.
So it makes me wonder, what exactly did Cyn promise J? Did she tell her that she could bring back Tessa? Promise to not let her die again? Something else entirely? Based on her comment to V, SOMETHING was definitely promised that ended up being a lie.
Everyone reacts to trauma differently. Saw a post a while back (will hyperlink it later if I can find it again) by another user doing a very good breakdown of how they perceived the drones' personalities changed as a result of their trauma.
N became a doormat because he kept getting hurt or hurting others every time he stood up for himself or someone else. (They also add that V's coldness towards him contributed to it and made him lose a lot of self-confidence.)
V became rather cold because she was basically forced to turn off her emotions to cope with what she was doing. And she was purposefully mean to N to chase him away as an attempt to protect him from Cyn.
J became a corporate bootlicker because she was terrified of being discarded again, so now she feels like she has to prove that she's useful.
And I think those theories all definitely hold some water here and help to explain why J still chose to side with Cyn, even after realizing that whatever their deal was was a lie.
It's likely that she realized that if she decided to side against Cyn, Cyn would've just killed her straight-up. And that's really not out of the question, considering what she did to N and V once she realized that those two had asked too many questions and realized that she was the bad guy they wanted nothing to do with. If J had also gone against her, she'd just be replaced with either a clone that had its memories wiped (as evident by Cyn's comment to N in Episode 7: "your clones will forgive me") or a personality shift, or just flat-out a completely different drone altogether.
There's also her comment towards V during their fight. "I promise it's better on the winning team.", which implies that J had full belief that Uzi wouldn't have been able to stop Cyn and Cyn was going to get her way no matter what.
That paired up with the personality shift might imply that she felt trapped. Cyn preyed on her vulnerability and fears of being discarded and weaponized them against her. She believed that Cyn was going to win no matter what, and if she went against Cyn, she was going to be discarded again. Despite knowing she was tricked, she probably stays loyal to Cyn because she's terrified of that outcome. She doesn't want to die, she doesn't want to be thrown away again. But the only person she feels she can get that security from is the same one that basically caused all of her problems. That same person is practically the equivalent of a god in the universe and seemingly NOTHING can stop it. What choice does she have in that perspective?
I still feel like she's an incredibly tragic character. Horribly mistreated by humans, forced to watch (or participate in) her best friend's death at the hands of the one she despises the most, sees the same bestie be horribly mutilated and disfigured, be forced to commit horrible atrocities herself, and be manipulated and lied to.
She deserves a redemption arc.
You're obviously allowed to still hate/dislike her, but that's just my thoughts on it!
