she did regret it though? It wasn't that she was taken too, but rather that she actually loves Leila. She knows they aren't going to kill Leila, but on the other hand, her brother has a high chance of dying, so she chose the outcome that had the least deaths as far as she could tell. its a bit more obvious in the novel where they actually have a whole discussion with her before she dies. But she was basically being threatened into the kidnapping. maybe she could have made better choices but she's just a teenager and its understandable that she didn't know what to do.
we as reader care more about the MC than unnamed side characters, but from Hannah's perspective obviously her brother is worth that risk. Hannah's death is supposed to be sad. it is a lot more jarring in the novel too and adds a good bit of angst there to transition into the next segment.
tbh i like that she died, but that is because i love angst, not cause i think she deserved it. I don't like how they are cutting out so much of the emotions out when making the manga....
honestly hannah deserved to die and lot of people in the comments are complaining that she was killed off but she initially is the reason why leila is in the situation shes in, it doesnt matter if she "regrets it" she still carried out the plan and she only started to regret it once she became a victim. she made her bed now she has to lay in it.