I think, that Hirata just knows, that he loses everything. His moneys, position disappeared, also his lies spill out. He knows, that this is end, so he want take his revenge and kills this bastard, who take everything from him and who he hates his half of life.
Indeed.... Plus what's better than to see your most hated person die a painful death.
That's why he took his time beating Yashiro up.
I think he hates Misumi more though
It s love/hate relationship.
Why did Yashiro choose this moment to die? Why choose this asshole, Hirata to kill him? When did Hirata become so emotional he let Yashiro manipulate him into beating him to death when only he knows where the ex merks took his money?