We Can't Tell the End from the Beginning
Which ending? Each of Cloud Atlas's six main plot threads has a different ending, and none of them gives us a real sense of closure. All except Adam Ewing's serve the purpose of leading us into the next story as we rocket backward in time from the future. Zachry's ancestors show us Sonmi's orison; Sonmi watches a movie about The Ghastly Ordeal of Timothy Cavendish; Cavendish reads Half-Lives; and so on.
One thing all the endings have in common is a general sense of victory or accomplishment. Cavendish escapes from Aurora House, Luisa Rey saves the day. The only exceptions are the stories about Sonmi and Frobisher. Both die (Sonmi is executed; Frobisher commits suicide), but not before each finishes a legacy that will live way beyond their years.
The end-end of the novel is an imagined conversation between Adam Ewing and his father-in-law. Ewing has decided to devote himself to the Abolitionist cause. His father-in-law is clearly the type of white dude who doesn't want to compromise any of his ideals in order to maintain absolute power. Ewing imagines his father-in-law saying, "[A]s you gasp your dying breath shall you understand, your life amounted to no more than one drop in a limitless ocean!" (11.15.11). Way to ruin any sense of accomplishment, dude.
Ewing then writes the novel's final line: "Yes what is any ocean but a multitude of drops?" (11.15.12). And that's the moral of our story. All of these characters are just drops in an ocean. But each one has left an impact; each one has rippled and affected others. Cloud Atlas implores us to live empathetically, to think of how our actions impact the world, to live thoughtfully and act—that's the most important—for the good of mankind. Powerful stuff.