Part 1: So the reason was because in the original’s chapter 52, Haruka laughed and mocked Haika. Although he was pissed, he still asked Shuu for his orders. Shuu replied with “Get him.” From these short texts, I analyzed that Haika didn’t want to take the violent route because she helped him. This meant that he tried to repay his debt. Unfortunately, Haruka went and pissed him off.
Part 2: Haika waiting for Shuu to tell him what to do is very important. Not only did it show who’s the top in the relationship, but it also showed that Haika is a rational person and doesn’t let emotions control him. Lastly, with Shuu standing to the side and letting Haika do everything, this showed that he wanted to let Haika repay his debt. With Shuu saying “Get him,” this meant that he was really pissed off from Haruka insulting Haika. He really didn’t like it one bit. A part in chapter 53 further cements this with Shuu saying that he would’ve felt pity, but now he’ll just enjoy Haruka’s pain. These are the reasons why I shipped them together. No, I’m not over-analyzing it. Shut up!