
I don’t understand what’s happening

Light hair is gay guy who dated a Senpai in high school but they broke up due to homophobic classmates. Dark hair reminds light hair of Senpai. Light hair fell in love with dark hair.
Dark hair likes girls. He envies popularity of light haired guy. Female coworker likes light haired guy and black hair is irritated by the gap in her treatment of them. He agrees to date light hair to make girl jealous. He is growing fond of light hair sincerely though. And because of his low self esteem and the story about the Senpai, he now thinks light haired guy only likes him as a replacement.
You are welcome.

I thought Hotta-san agreed to go out with Kashiwagi-kun because he wanted to give him a chance? Hotta-san being spiteful to Hazuki-san was just a byproduct. I'd be pissed off too if someone all of a sudden changes their attitude towards me when I didn't do anything wrong. They may not have to date, but she didn't have to keep them from being friends.

You've got it backwards.
Hotta was going to reject him. ( http://www.mangago.zone/read-manga/kirakira_kun_to_imaichi_kun/hs/mpn_chapter-1982007/pg-8/ )
Then Hazuki showed her double standard again. ( http://www.mangago.zone/read-manga/kirakira_kun_to_imaichi_kun/hs/mpn_chapter-1982007/pg-10/ )
Pettiness surfaced. ( http://www.mangago.zone/read-manga/kirakira_kun_to_imaichi_kun/hs/mpn_chapter-1982007/pg-12/ and http://www.mangago.zone/read-manga/kirakira_kun_to_imaichi_kun/hs/mpn_chapter-1982007/pg-13/ )
And he staked dibs on the Kashiwagi in spite. ( http://www.mangago.zone/read-manga/kirakira_kun_to_imaichi_kun/hs/mpn_chapter-1982007/pg-14/ and http://www.mangago.zone/read-manga/kirakira_kun_to_imaichi_kun/hs/mpn_chapter-1982007/pg-15/ )
Notice the throb panel here. That's guilt. http://www.mangago.zone/read-manga/kirakira_kun_to_imaichi_kun/hs/mpn_chapter-1982007/pg-17/
And it continues on the next 3 pages.
Chapter 3 is when he begins to grow affection for Kashiwagi. Chapter 4 is when his spiteful feeling are just becoming more explicit as Hazuki becomes openly hostile.
The basis of it all is Hotta's low self-esteem. From chapter 1 being jealous of Kashiwagion the train to Chapter 4 where he assumes he's a replacement for the senpai. None of his actions so far were first done out of love or concern. It was all to make him feel better about himself and fill his void. :/
I’m gonna end up flipping a fucking restaurant one of these days due to this frustration (╯°Д °)╯╧╧