Norman Bates has dissociative identity disorder and splits into two personalities - his normal self and his mother. He becomes jealous and obsessed with his mother when she starts dating again. This drives him to kill her by releasing carbon monoxide while she sleeps. He then keeps her mummified body in the basement and talks to her in his mind, taking on her personality to escape knowing she is dead. As a child, Norman experienced trauma and abuse, and his mother was the only one who protected him, leading to his splitting into her personality under stress.