You know, if they had all banded together, they could have forced Shimada and asshat to leave them alone and probably let Umekichi live.
Shimada and asshat have said several times that the want reinforments, but they aren't coming. They are being kept in locked down, because if they go out to a brothel, they might get killed, which means they are at least somewhat important to their plans (though only Shimada's boss know how critical their job is/was.) So if they had said something like: "Nope, we ain't gonna kill one of our comrades. Sure, rule 2 was that if you don't follow an order you die, you can go ahead and kill me. Kill all of us, and then what? What about the reinforcement we never got? Who will fight the Satsuma cunts in our stead? You two? Against 20+ guys? You leave Umekichi alive and we will still fight for you when you need us."
Though they probably would have gotten poisoned afterwards. Who would trust guys that defy your orders?
Anyways, you always have a choice. No one can force you to do something you don't want to do.