Shōgun

I sort of read it differently. Presuming Toronaga was correct in his observation with Mariko at the end of the episode, Hiromatsu effectively threw himself on a grenade to prevent the rest of the generals from killing themselves in protest. I don't think Toronaga wanted Hiromatsu to die originally, but he also couldn't deviate from his deception of surrender and had to let the thing play out.

I think it was meant to show how cold and calculating Toronaga really is. This wasn't just a retainer, this was Toronagas best lifelong friend.

The show always paints Toronaga as the sympathetic figure and Ishida as the "bad" guy. But I think at the end it's meant to flip the script. I believe it reveals Toronaga has always been angling behind the scenes for the power of Shogun. He most likely did commit the crimes the council wants him dead over. Ishida really is trying to keep the council of five in power but Toronaga outplays him with these maneuvers. Using Mariko to turn the heir's army to his side ensures the defeat of the council, and right after that the defeat of the emperors forces as well.

Toronaga assumes total control and only allows the emperor to live as a captive. Even though it is not revealed until the last scene I think it was implied that Toronaga was always angling to be the Shogun, and really is the "bad" guy from the Imperial perspective.