DON'T READ THIS IF YOU HAVEN'T ALREADY READ THE QUIET AMERICAN: I hate to be responsible for giving away endings.
In The Quiet American, Pyle, like Don Quixote and Madame Bovary, has read the wrong books, and thinks reality should match what he's read in books. Fowler is like Rick in Casablanca: he believes he is neutral, but he learns that he cannot be uninvolved.
After witnessing the atrocities of war, after seeing a woman covering her dead baby and a man cut in half by a bomb Pyle is responsible for, Fowler recognizes that he must act. Because of the damage the well-intentioned Pyle has and can cause, Fowler acts to have Pyle killed.
Pyle is willing to aid atrocities for what he sees as a greater cause. Of the deaths of civilians after a bombing, he says "They were only war casualties. [...] It was a pity, but you can't always hit your target. Anyway they died in the right cause. [...] In a way you could say they died for democracy." The individuals that were killed were simply collateral damage for Pyle in the greater cause of bringing democracy to Vietnam. And for this naivety and the damage his innocence can cause, Fowler is complicit in his murder.
And yet, Fowler's action is not terribly different from Pyle's action. Pyle is willing to sacrifice lives in a utilitarian ethic--their lives will bring about a greater good. Fowler is willing to sacrifice Pyle's life in order to save other lives--to bring about a greater good. That we can look back and say that both men failed in their goals (Pyle couldn't bring democracy to Vietnam, Fowler couldn't prevent the terrible violence and suffering that came from American involvement in Vietnam) can make us question the efficacy of violence as a means to an end. But while we may sympathize with Fowler's action more than Pyle's, they both acted on the same principle: life is expendable if it is for a good reason.
Fowler recognizes his utilitarian assessment, too: "what right had I to value her less than the dead bodies in the square? [...] I had judged like a journalist in terms of quantity." And he recognizes his similarity to Pyle: "Was I so different from Pyle, I wondered? Must I too have my foot thrust in the mess of life before I saw the pain?"
It is a beautifully written and incisive book.