I think Izumi's Dom tendencies go a bit more complex than that. I read another sub verse yaoi that explained it better. The Dom seme would be very similar, they would do everything for the uke, all the household chores, gift them everything they wanted, plan their entire life around the uke's preference. Not because they are a very nice guy or a doormat, but because their Dom tendencies required them to make their sub so absolutely comfortable and dependent on them that they could never live without them or even think on leaving them. Its love bombing and encouraging absolute co-dependance, which is of course in no way healthy for a real relationship, but very much par for a subs verse dynamic. I think Izumi's love is something very similar here. He wants to absolutely love his sub to the point they cannot live without him, but he requires absolute commitment in return or he goes off the rocker. I think at the beginning he had pretty much giving up on achieving as close a bond as he needed and had spiraled into self-hatred (convinced he's one of the nastiest Doms that exist and that his tendencies and needs are to much to bear by the people around him), ergo, cue the nasty behaviour at the beginning. When you are convinced you are not worth it, you act like it because you don't care to even try.
Why make him a blackmailer and two timer at the start?? Could’ve found another reasoning for the forced proximity and living together while also still having him be the delinquent type without doing really icky stuff, cause at the end he’s adorable with the “ohhh I’m so dark and evil I wanna control by showering the person with a lot of love and just wanna cuddle someone to sleep and be loved back” YOU CUTE FUCK
“If only Izumi had the shred of decency this guy has” BITCH IZUMI ASKS IF YOU WANNA PLAY OR NOT AKSS IF YOU WNANA HAVE SEX OR NOT EVEN ASKS AGAIN BEFORE GOING IN AND TAKES A NO WHILE THIS ASS WAS ABOUT TO JUST COMMAND U AGAINST YOUR WILL BRUV PLS BE FR, JUST CAUSE HES ON HIS KNEES RHE NEXT DAY DOESNT TAKE IT BACK