I don’t think the premise is unconscionable. In law enforcement, it’s well known that some women are attracted to men who they know to be serial killers, violent criminals. I just finished reading a book on the serial killer Ted Bundy who confessed to killing 30 women. At his trial, the front was filled every day with “groupies”, young women attracted to him because of — not in spite of — his pathology.

Go figure.

In this case, the serial killer is a woman. — that’s rare in real life, rarer is reel life. Almost all serial killers are men — of women, btw. The very few female serial killers are killers of men, their murderous compulsion arising from a history of physical and sexual abuse at the hands of men. So in this premise the serial killer’s lover would come to have more to worry about than his conscience. He should also have to worry about his own life, that he’ll be her next victim.

However, “music” doesn’t contribute anything to suggesting or enhancing his internal conflict. But something like “milquetoast” would because it suggests he’s lucked into a sensual relationship he never had the macho seek out for himself. So:

“When a milquetoast teacher discovers his new girlfriend is a serial killer, he must decide whether to turn her in or continue to enjoy amazing sex.”

Now my usual m.o.would be to pounce on “decide” because it’s not really an objective goal. Protagonists are not supposed to decide. They are supposed to do. But in this case, the conceit of the story does not entail an end of Act 1 call to action. It entails the guy getting involved in a relationship. The hook of the story is that after he’s hooked on the sex, he discovers she’s a serial killer. Which raises the dramatic question: does he give up the greatest sex of his life to the police?

Now the fun really begins, watching the MC agonize over his, uh, horny dilemma. This could be a dark romcom story. Even if it’s played straight, not for laughs, but for murderous suspense, at its core it’s a romance, a love/lust story.