Interesting Musical Choice

So I watched The Killing Fields for the first time a few weeks ago, and really enjoyed it, but that's not the point. On second viewing, I noticed the soundtrack choice in one scene in particular. In the scene where Schanberg is alone, back in the States, going through photographs, walking around his house, he turns up his stereo, which is playing an opera, while on his tv he watches an old newscast of Nixon talking about the invasion and some images of the fighting and violence. Meanwhile, the aria in the background reaches a crescendo. Here it is on youtube:
http://www.youtube.com/watch?v=drAnr9a1fRk

.... Still with me? Anyway, I thought that particular song was really familiar, so I looked it up: "Nessun Dorma" from Giacomo Puccini's Turandot. The opera is based on a Chinese fairy tale about a prince who wins the heart of a cold and bitter princess. That particular song is one of triumph, as the prince who sings the song basically says he will win the princess's heart. (I could go into more detail about the story of the opera, but that's also not my point here).
So I wonder at the choice of this particular aria for that particular scene in the movie. Not that I think it's a bad choice, but it's definitely an interesting one. It had been a while since I'd heard that song, so I wasn't sure what it was about, and based on the scene, I kind of assumed it was a song of despair. But the music that accompanies the lyrics is triumphant and joyful even by itself.
It seems weird to include the song since up to this point, the movie has been about how Schanberg's attempts to help Dith Pran have failed. But the powerful, emotional crescendo seems oddly appropriate for what's going on in the scene. What do you guys think?