Responses
It's like a payback. They both known each other for a long time. Hirata got stabbed on the side then he stab Ryuu to get even. Like Ryuu said, "You couldn't give up, could you?" Like Hirata couldn't leave in such short notice without doing anything to him. If he killed him, it could feed evidence. So it'd be pretty bad if he did it when the cops arrived. Sides this stab Hirata did pretty much killed him anyways. So he probably wanted Ryuu to suffer instead of dying in a quick death.
So Ryuuzaki and Hirata stabbed each other?