I would want to respond to this question by focusing on the way in which the various characters change in their thoughts regarding death after 9/11. Let us remember that the author is using these characters to present America as a whole, and so examining their changes can reveal the changes that DeLillo believes occurred in America as a whole.
Keith, for example, is haunted by the image of death. This is something that other survivors experience as well. Keith moves from a fear of death to a philosophical pondering about the meaning of life. It is clear that after the destruction, Keith enters a different world, and with his new knowledge and experience, he feels isolated and misunderstood by those around him, even his wife. Keith is now shown to occupy a strange existence in a kind of limbo that separates him from other humans. He has become aware of his own inherent fragility and has been profoundly changed and humbled by his experience.
Perhaps we can therefore use Keith to comment on the way that DeLillo believes America, post-9/11, changed. America suddenly was forced to realise how vulnerable it was, and it changed forever, trying to process the terrorist attack that struck at its very heart and finding itself asking big questions about life and death that it had never had to do before.