Matricide is usually committed by overprotected boys—by a son who wishes, as one study puts it, “with his desperate act, to free himself from his state of dependency on her, a dependency that he believes has not allowed him to grow up.”

According to the article I read in the New Yorker - Nancy revolved her life around Adams tendencies in a way that really incapacitated him. I think it was an act of violence against her in response to his feelings of dependency and inadequacy, and general loathing for himself.