I feel seijuro deserved death. He had already left the way of the sword and had stopped training altogether just relying on his genius talent and previous muscle memory. And years of loafing aroumd in the pleasure quarters too dulled him I think
Damn, Seijuro was a top predator and he was killed like that. I actually like him more than Inshun and Kohei, and they both lived. Seijuro died like an easy prey being cut brutally like that.