There's a thing called pathological jealousy is a delusional jealous one had over their wife. They kept thinking that their spouse is cheating and if I'm not wrong it's a mental illness. You could have the main character have delusional jealousy towards his/her wife or fiance causing him to abuse his/her wife or causing him to kill whoever had contact with the mc wife. You could end it with a happy ending where the mc met a psychiatrist and cure his delusion or a sad ending where his wife left him or a plot twist ending where those delusions are true and her wife is really cheating or a tragic ending where the mc ends up in the mental health care or something. You can make the mc have some backstory why she/he develops the mental illness. Well, this is just a suggestion, and it's your story do whatever you want. Good luck :)