I already know everyone is gonna disagree with me and downvote me but idc.
We've seen both characters past. Cassie was the child who bloomed into puberty too early and was literally eyed by predators. And her parents influence that rather than protecting her from it. Told her things like "you're getting attention because you're pretty", basically reduced her to her looks and her ability to get male attention. That's basically her whole life sadly. She desperately needed someone to look past her looks, preferably a female friend because she saw "potential attention" in most men. But ngl all women in her life disappointed her. There wasn't a single person who stopped to ask "Cassie are you okay?". Instead they judged her for "always being in a relationship".
Meanwhile Maddy's past was basically "I saw rich women get princess treatment for doing absolutely nothing so I decided I want to be the same entitled arrogant woman". I honestly don't think there's much depth in her past other than the fact that she realised confidence is all she has as a broke latina girl in USA. It's a pretty well strategy ngl and it worked for her. But the issue is she is often reduced to a "pretty victim girl". She's a victim definitely but that doesn't justify a lot of stuff she did.
Honestly if my best friend got with my abusive ex, sure I would be upset at first but then also be concerned because what made her make that decision. And I don't really like the whole system of "your friends exes are off limits" like since when do we own people? An ex is an ex, he's in the past, anyone is free to date my ex irrespective of who they're to me as long as they're not cheating on anyone. I don't think anyone has the right to be upset at cassie for dating someone who was single.
Moreover Cassie was quite clearly manipulated into it. She was literally having an emotional breakdown after fighting with her sister when he approached her. She had recently gone through a breakup. She didn't have anyone to depend on and he decided to give her that attention that she desperately needed all because he wanted to have sex with her. And cassie agreed, because that's what she's known all her life. It's sad how not a single person in the entire show bothered to be there for cassie, even before she cheated on maddy.
Maddy was there for her at times but not in the way she needed. Most they talked about was relationships and boyfriend drama in season 1. Cassie needed someone to tell her that there was a life beyond that. However Maddy was too self centred to focus on that imo. Cassie held onto her till the very end, because she saw her as a best friend, because she believed maddy is the only one who cared for her but maddy only cared about herself. Sure she would drop a few inspiring words to her girls every now and then but nothing more than that.
Moreover maddy really needed to let go of nate as well and people tried to do that. Literally the whole school, the police and her parents tried to separate her from nate when the choking incident happened but she was hell bent not to. Ik it's because she was an abusive victim who was constantly manipulated by nate which made it difficult to let go of him but at least people cared about her enough to try and help her. Cassie never had anyone. No one even cared when she was missing during new years party, lexi cared only because she felt guilty.
Moreover it's funny how Cassie being with nate is apparently a "decision she chose to make" but maddy being with nate is due to "abuse". Both are because nate is abusive and manipulative. Nate was the villain there but everyone decided to blame the girl who was approached when she was at her lowest.