Symbolism, Imagery, Allegory
There is plenty of discussion in The Rules of Survival about people falling victim to their demons. This is especially relevant when Matthew talks about his mother—much as she's a horrible person, in retrospect he feels like she cannot help her behavior. She's drawn to doing awful, abusive things because she has these "demons" that live inside of her:
I believe that I could and should have known about the demons that were on the borderline of ruling our mother. I had actually seen something in her eyes and felt some force in the air around her for many years. (19.5)
These demons aren't the kinds that show up in The Exorcist, of course. They are just Nikki's own personal issues and personality traits coming up to surface. She cannot control her more erratic and violent urges… even when she tries to. Matthew understands this about her, though it doesn't mean that he can forgive or even love her. As much as her children are victims of Nikki's demons, then, she is a victim of them, too.