A large part of why Endo is watching his running mouth is bc if he doesn't he knows it could cost him Sakura being willing to associate with him past the battle. Keeping his promise to not touch Furin again is one thing, it's another if Endo makes it clear just how much he wouldn't have minded destroying the town Sakura cherishes. So then Sakura would be less likely to hit him up/respond to his messages. So I'm glad Endo knows he's still on thin ice bc yes sir, you are still an ass!!!.