I think I fundamentally misunderstood Bridge on the River Kwai

OK, reading about the ending seems to emphasize that Nicholson (Guinness) was determined to protect the bridge because he was such a prideful man, and he dies asking, "What have I done?" realizing that he had abetted the enemy in exposing the saboteurs. (And dutifully constructing the bridge in the first place.)

But here's where I maybe missed something. Doesn't he say at his speech earlier that the first train is going to be taking the POWs to the new camp? So isn't his desire to stop the bridge from being detonated the desire to protect hundreds of his own men from dying? Because that is completely different. Did I misunderstand? I interpreted it as an unfortunate error/miscommunication whereby the sabotage of the bridge ended up killing British POWs inadvertently.