Is It Normal To Want To Kill Your Girlfriend?

Like it's not some necrophilia or something but I often think about stabbing her in her sleep and for her to wakeup and see the person she cares about most trying to kill her. I want to see that face she makes realizing my betrayal of our love and the confused look in her eyes. I wonder if she'll have the guts to hit me back and try to escape while I play around with her blood and insides and cut open the heart that belongs to me. I think about how I'm going to get rid of her body, how I'm going to have to avoid not being caught, how I'm going to kill myself if I ever do, how I'm going to deal with loneliness of not having her around anymore if I don't. I love her so much and she's the most important person in my life, but I just think about this every night. I can't fall asleep for hours imagining that expression of fear and confusion she’ll make if I stabbed her one day. Is it normal to think of this obsessively?