'Imagine' an irony?
Most sure that was not the intention from the director or the writer, but hearing Lennon´s "Imagine" at the end of the movie, can be viewed as something pretty ironic, even sarcastic. Specially when you think about some lines of the song. Consider:
"Imagine... no religion... no possessions...no need for greed... all the people sharing all the world..."
Well, if you think about it, that was pretty much the kind of society that the Khmer Rouge were trying to create in Cambodia, the society from which Pran had just fled.