I think she is his trophy and he wants and needs to isolate her from others to keep her only for himself
From my pov though he's reluctant to admit it he's jealous of other "normal" peoples emotional attachment. He doesn't like feeling left out and he takes the sentimental possessions from the victims to have superiority over them. He feels like the teacher is an already vulnerable person that is very understanding of others... i think he doesn't want to be lonely and attached himself to her. Hes now going down the obsessive root where hes going to take her as the item from everyone else as they don't deserve her. I dont think he realises she's probably kinda crazy for her loved ones.
i’m not sure what his objective is as of yet. it seems like he wants the teacher as a trophy and yet i feel like it’s not that simple…but the way he’s isolating her and ruining her life i don’t know. this is gripping though. and the art is great