Some people have a fantasy about rape.
I think it's fine as long as it doesn't go to the extreme.
Some men/women have a fantasy of rape where they pleasure the female/male against their will. Ok. Whatever, just don't do it in real-life.
I think it might get serious if the fantasy involves beating, torturing, cutting up, etc. Not saying you will actually commit a rape but you will probably hit your wife and you should get help before you become an abusive husband.
You said you get off on murder too, therefore I would say you should go see someone.