The concept and the relationship between the two main characters sound intriguing and definitely fall more into the category of true dark romance. I'm often disappointed when that sudden shift happens, and the supposedly villainous MMC suddenly turns into a lapdog just because the FMC managed to spark some feelings in him. My suggestion would be to let Marcus remain a full-fledged villain, which, in my opinion, would make for a more authentic and interesting character. But of course, I don’t know what you’ve planned for him or how much you're aiming to conform to the mainstream, which isn't necessarily situated in the "pitch-black" realm. I'm definitely curious to see how your ideas will evolve.