because a teenager had to get decapitated to satisfy a 2000 year old loli with "stockholm syndrome".

Really don't think she enjoyed being with Fritz in any way. Since Ymir was a kid, she felt benevolence was the only thing missing in her life and as a lonely slave there was little she could do to get it. She freed the pigs to realize if she's capable of giving it, she's very well capable of receiving it. She could have snapped Fritz at any moment but she kept giving him chances of reciprocation and when she realized he won't give her what she wants, she gave up on herself, believing she will never be loved.

Much more than Stockholm syndrome, I read tolerance and patience here. She just got stuck with Fritz, she would have reacted to anyone else the same way.