There are plenty of arguments to be made that Dick is bad for Nicole and that he manipulates her and exacerbates her mental illness. He can’t stop thinking of her as a patient and whenever he does something wrong and she calls him on it, he more or less accuses her of being crazy. She encourages him to work on his career but he uses her as an excuse not to, keeping her in a state of constant guilt. She finally seems to get better when Dick is out of the picture.
Likewise, it can be argued that Nicole was bad for Dick from the start. From that perspective, Nicole seduced Dick and then became so dependant on him that he didn’t have time for his brilliant career.
Devereux is our star antagonist. He rapes Nicole when she is eleven or twelve or thirteen years old. It can be argued that he is the root of both Dick and Nicole’s problems.
Rosemary relentlessly pursues Dick, even though she knows he’s married to Nicole. Her relationship with Dick causes Nicole lots of pain. Still, she’s presented as a fairly sympathetic character, and a big chunk of the novel is narrated from her perspective.
By treating Dick like a possession, and treating Nicole like a crazy person that she needs to buy a husband for, Baby contributes to Dick’s eventual downfall, the break-up of the Divers' marriage, and possibly to Nicole’s poor mental health.
Elsie pushes her daughter to pursue Dick, knowing he’s married. She seems not to care that both Rosemary and the Divers might get really hurt.