He sounds very insecure and definitely some degree of bi/homophobic. Remember that anger always comes from a place of fear or hurt. He’s letting his own fears and insecurities impact how he is reacting and treating you, which is not okay. How long have you been together? To me, This doesn’t seem worth breaking up over as long as he is willing to discuss this with you openly and rationally. I suggest asking him sincerely about what he is worried about or why it upsets him so much, and reassuring him that these fictional characters have no relation to your partnership. If he isn’t receptive to this and isn’t willing to accept it, then perhaps he isn’t right for you. I don’t believe in just throwing people away without giving them a chance to change first, but he realllly needs to make an effort for himself and for the relationship to get over these insecurities that are probably linked to some level of bigotry. If he loves you, then he needs to love and accept your sexuality and your creativity.
I suggest you dump him and find you a nice girl who understands the concept of fiction.
Honestly he is being absurd, biphobic, and weirdly controlling. Wanting you to kill off one of your characters so that her partner ends up with a man is…I don’t know what it is but it’s not a good sign.
It just sounds like your boyfriend doesn’t like that you’re bisexual.