DO NOT READ IF YOU HAVEN'T SEEN THE ENTIRE MOVIE--Question about the end

It's left intentionally ambiguous.

The whole point of the story is to discuss what morality makes up society, and if there are any limits on that morality.

Leaving the ending ambiguous means there's no neat "it was Right/Wrong" message from the film. You're meant to draw your own conclusions.

It's a morality exercise, where you can't use the ultimate outcome to justify your conclusion. That is, you can't argue "the ends justify the means" or "the means can't justify the ends".

The hidden fourth bomb explodes in California. You can tell from the license place of a taxi cab. So all the characters in the movie die.

In the extended version the fourth bomb is placed in the same room as another bomb. He tells them about the bomb kept openly visible and they diffuse it and start celebrating. What they do not realise is just behind them hidden behind the door is the fourth one. As the bomb counts down to zero the movie gets over.