That probably was the reason ... than the "main characters".
I'd argue this is just rising conflict and basic story telling. Given how prior to this you literally have people going "Well he's bad but he's not abusive" and a bunch of other people saying "Uh, no he's already abusive" shifting it to showing how his psychological abuse can change to physical at a drop of the hat because he has a fragile ego. Hell, you're not even reading what happened in the backstory correctly. Like
"he has no respect for women since they were all over him once they decided he was hot. But it's not like Midori was after him for that reason, so it almost seems like a pointless addition."
is wrong. Chances are sure some women actually noticed him because he looked more handsome now, but it was him being a misogynistic shithead that leads to him thinking that's the only reason why women care about him and that women are his little play things to manipulate and use. Hell he probably thinks Midori only cares about him because he thinks all women are easy to manipulate and are shallow.
It's funny too, it doesn't remove tension because Midori is an abuse victim You're aware abuse victims can literally die before they really want to leave their abusers right? Hell, it's an incredibly tense chapter because with how it leaves off we're left going 'So how is Midori going to rationalize her abuser's behavior to herself and will it be the wake up call she needed? Is Tazune going to get worse?' and I fail to see how that's a failure to story telling.
Or if you're complaining about how this resolves the tension by saying 'Oh well now it's clear as day Midori will leave!', because if so, I don't know if we're reading the same series. It was obvious Maki and Midori end up together. It still focuses on and is about Midori and Maki, hell this chapter ends literally going back, unless if you somehow think that a perspective shift and a bit of history somehow makes this story about him, in which case I don't know if you've ever read a narrative with more than two characters and is more than a few chapters long. This stuff happens all the time in media so I really don't follow your issues here.
Actually. rereading.. some of my assumptions.
I feel like its a matter on picking up on red flags and reading into it because of those flags. Like someone pointed out before, prior to chapter 5 he was clearly an abuser if you knew what to look for, chapter 6 just cements that by showing how he's not just a manipulative shithead but he's a ticking time bomb too.
Don't feed the trolls.
Thank you for reminding me, Heavensrun.
Also thank you Blaastar in also noticing he's a PUA and not an incel :P