if you're talking about how cold she became, I think it's because as you mentioned, ml became warmer. She felt comfort that ml was also suffering and that they were suffering together. But he became more human and she felt left behind? she mentioned that they were both monsters. Time was still for her and it became evident bc ml and fl were both growing physically (and mentally for ml).
The person who killed her was ml's mother, but she couldn't blame him because she understood that it wasn't his fault. She tried to blame fl because he turned ml more human, but she liked fl. She didn't know who to blame for her misfortune, in a way I felt it was kind of similar to what fl had to go through in the beginning.
It might also have to do with the contrast so that it will be balanced and not both characters being cold which could obstruct the storyline itself so it could be another reason for such
The manga is great overall in my opinion. Since the ML’s character development is great and in contrast to Tori’s character development Tori became more cold and distant whereas the ML became more warm and kind towards the FL. The character design is also very nice but what happened to Tori in the end did anyone else find it confusing as to what happened to her to cause such a drastic change ?