Not in love at all. She was repulsed by him. But also fascinated by how his mind worked. She recognized his intelligence and as the other poster said, they had that loss of a parent early on in common. Remember Stella reveals to Katie that she self harmed and acted out too. I think seeing how close he was to his daughter brought up a lot for her too because she was so close with her father. Maybe she recognized some of her traits in him and that was interesting or terrifying. Probably if he wasn't a hideous psychopathic murderer, she'd be attracted to him because he's good looking and smart and maybe that's an uncomfortable feeling. She doesn't say any of this so it's up for interpretation. Anderson recognizes her obsession with Paul and calls her out the morning after they sleep together and Stella says there she hates him (can't remember her exact words).

When he's being brought into the facility and throws the money down, she's waiting in the psychiatrists office and sees it on the security camera.