I think Hirata killed the Gouda group leaders in order to kill two birds with one stone. He gets to remove the people who he is indebted to due to the failed drug deal, and gets to frame Yashiro for their murders, possibly getting retaliation from the Gouda group and maybe even the police. The Sanawakai group is new, the Gouda group were apparently trying to join their branch so I don't know if they would really care about the leader getting killed seeing as they weren't "family". Also, Misumi who is Yashiro's "group dad" is friends with their group and his step son is friends with their young leader, so their doesn't seem to be any personal vendetta there. Maybe I've got this wrong though, as I do think now this whole drama is more than just Hirata'a power struggle within his own group. Maybe the men who killed the Gouda group weren't even doing so on the orders of Hirata?
This is why I should read things in full first, rather than just the raws. Anyway, it does seem that Hirata ordered the deaths of the Gouda group in order to create conflict with the Sanawakai, not sure about this seeing as they weren't formally affiliated. It does mean he gets to target and frame Yashiro, although there isn't any evidence. The question should maybe be then despite this, what motivations does the Sanawakai have to target Yashiro given the apparent close relationship between them and Yashiro's main group. The debt is Hirata's after all, and that still needs to be paid. Surely the trail leads back to him, rather than Yashiro's sub group, idk.
Still don't get which group is which and who's who ╮(￣▽￣)╭ But this Hirata dude pisses me off