The ending of In Cold Blood, in brief, is this:
After Dewey flashes back on the execution of Dick and Perry, he flashes back again to an afternoon the previous May, the day he feels the Clutter case truly ended for him. He'd gone to weed around his father's grave, and reflecting again on his life and family, he notices the grave of the adult Ashida daughter, killed in a car wreck.
He then sees the now adult Susan Kidwell, Nancy's best friend, who's visiting the graves of the Clutter family. Dewey and Susan exchange small talk: Bobby Rupp's just married a beautiful girl, Susan's in her junior year at college. As she hurries away to meet a friend, Dewey thinks about what Nancy might have looked like at that age.
Then he walks away under the big sky.
This ending, which even Capote finally admitted never actually took place, is open to a lot of interpretations. One of them would be that time passes and tragedies recede into the past. Bobby grew up and married somebody else; Susan went on to college anyway. The problem with this kind of interpretation of the ending is that it's hopeless and fatalistic. Perry and Dick were right: rules are meaningless, do whatever you want, nothing matters.
But that's not the tone the author seems to strive for at the story's conclusion. It's true that life goes on and grief fades, but the mood of that last scene at the cemetery is more wistful and philosophical than hopeless. Not to get all Zen on you, but life began before you, and it will continue after you. We seem to be talking Grand Scheme here. Life isn't hopeless, dude, it just is.