I suppose the best way to open this answer would be to quote from the Joker in The Dark Knight:
And here we go...
I would sumbit that it is the ending that is the most challenging element in Hamid's work. Part of the reason is that Hamid does not construct anything in terms of certainty of the ending without the reader's understanding. Certainly, we have Changez's insinuations that the American is "on a mission" and that Changez is a target. We also know that Changez might be paranoid. We also have some perspective that perhaps Changez has moved from words to action and is planning something to be done. In the end, as they both leave the restaurant, it is certainly conceivable that one of them is planning something or both of them are not planning anything or that neither one of them are planning anything. We simply do not know from what Hamid has constructed for us. Changez's lines at the end are haunting to this point:
It seems an obvious thing to say, but you should not imagine that we Pakistanis are all potential terrorists, just as we should not imagine that you Americans are all undercover assassins.
In this, one is left to straddle if Changez is speaking a universal truth that he understands and embraces through his own experience or if he is speaking "fundamental truths" in a cynical manner to undermine them and gain supremacy. We are no closer to understanding anything when Changez suggests that the shiny element from the American is a "business card holder." When Changez speaks of "intimacy," it could be a statement of solidarity or a statement of joy in that he takes special pride in being able to eliminate someone from "the other" that he actually knows. We are left with our own preconceptions to assess the situation. Hamid knows this and has done this deliberately:
I certainly was working toward an ambiguous ending, one that would reflect the reader’s own view of the world back at him or her. Depending on how the reader views the world in which the novel takes place, the reader can see the novel as a thriller or as an encounter between two rather odd gentlemen. Because the journey I am asking readers to undertake is emotional and troubling, I knew I wanted a strong narrative pull, a mystery that would add urgency to their reading. The ending, I hope, is the culmination of those efforts.
The ending to the novel is not about the characters. It is about us. It is about how we view the issue of fundamentalism and terrorism in the modern setting. It is about how we have these cultural predispositions and through dialogue and narrative exploration, as Changez has demonstrated through nearly 200 pages, we can seek to better understand these biases. The ending is reflective of our own interpretation and our own understanding of self and our world. It is to this end that Hamid has given a work in which the most profound understanding that emerges is one about ourselves and our own view towards the issue that is one of the most definitive in and reflective of our times.