what finally brings dewey that closure he needed in the clutter case?

The final episode, with Alvin Dewey in the graveyard, is one of the only fictional scenes in the book. Capote wanted to end In Cold Blood on a hopeful note, and so he invented the exchange between Dewey and Sue Kidwell (a move that earned him a great deal of criticism, from reviewers who otherwise applauded the book). Nevertheless, Capote wanted to use this scene to make a final point: that life persists, and time marches forward, even in the aftermath of such great upheaval and tragedy. Both Dewey and Sue Kidwell are in the process of looking back, but they are also oriented toward the future. Even the landscape, with which Capote began the narrative and with which he now concludes it, offers a poignant reminder that the seasons – like human life itself – come and go with the passing years.