As flashbacks throughout the movie showed, Bonnie was a cruel stepmother to Alex, with the first showing her slapping Alex for not mowing the lawn. Another showed Bonnie walking in on a younger Alex as he was getting dressed in the bathroom wearing only a towel, mocking Alex for his weight and ordering him to refer to her as his mother, later snarling at him to "get over it" when he stated his mother was dead and shoving him to the ground when he attempted to force her out.
In a flashback to more recent events, Bonnie was shown packing for a trip she was taking with Paul, with her conversation with Alex when he walked in revealing that he was being kicked out of the house. After admonishing Alex for being at fault for being evicted, Bonnie went to reveal how unsatisfied she was with Paul and felt he wasn't sexually attracted to her anymore, while accusing Alex of lusting after her. Alex revealed to Bonnie that he wanted to remain in the house, with Bonnie responding by claiming his eviction would be "doing him a favor" before accusing Alex of not being a man and revealing the truth behind him being kicked out: Paul was initially against sending Alex away, only for his wife to convince him by claiming Alex made a pass at her.
Bonnie once again accused Alex of being attracted to her and subtly invited him to join her in the shower, only to reject him when he kissed her and callously stated she "wasn't that desperate". Alex responded to his callous stepmother with anger, proclaiming she wasn't his mother when she referred to herself as such and referring to her as a "slut", returning the gesture when Bonnie slapped him in response. Alex then began kissing Bonnie again, which led to a tryst between the pair. When Paul arrived and caught the two together, the evil Bonnie proclaimed that Alex raped her, with Paul (having resented his son due to blaming him for his first wife's suicide) believing her claims and throwing Alex out of the room. Afterwards, Bonnie attempted to convince Paul to have sex with her, while putting up (likely false) resistance to his intentions of reporting Alex to the police. Shortly afterwards, however, Bonnie and Paul were both murdered by Alex, who stabbed the two to death with a dagger from his bedroom.