The Bridge on the River Kwai ending explained

What happens

Nicholson spots the commandos' wires and raises the alarm. Wounded, he recognises Shears, mutters "What have I done?" and falls onto the plunger, destroying the bridge and the first train. He realises, too late, that he has been helping the enemy.

The last line

"Madness!"

What it says

Nicholson's honour, discipline and pride started as virtues, but his pride in the work swallowed its purpose. Clipton's cry, from a neutral doctor, names the cost: rigid duty can defeat itself, and the ending lets you feel that without a lecture.