Yagami wasn’t responsible for Shigure’s death. He only cares for power. He hasn’t made it to be the group head, though. The group head is that Mihara guy mentioned in chapter 6. Originally Yagami wanted to make Yukitaka his ally to aim for that position or at least to get a higher position under Mihara. Yukitaka agreed to make a deal with him to set Haru free. Yes, he was an asshole to Haru, but only because he knows Haru is capable and wants to have him as his underlings. It’s expected that as yakuza they don’t let people “leave” that easily. He didn’t force Yukitaka either. That guy didn’t expect that Yuki would side with him in exchange for Haru’s freedom.
I really like this yaoi, but am I the only one who's upset that Yagami got exactly what he wanted? He was responsible for Shigure's death, was a manipulative asshole to Haru and essentially forced Yuki back into the Yakuza life because of Haru and he got to be the group head. He not only didn't get any retribution for his actions, he got exactly what he wanted. It really bothers me.