Because it's not guaranteed that they will let him live. When Oda literally starts knocking on the castle gate after killing everyone, Nagamasa might just go, "Alright, I'm gonna die anyway. Is Oda's chef still in his prison cell? Good. I'm just gonna kill him out of spite. Guys, don't tell the wife."Why would he do that.... He knows how the fight with Nagamasa will go, so just sit still and wait.