We're going to go out on a limb here and suggest that there are not one but three endings to the novel. The first ending is when Christmas dies, the second is when Hightower dies, and the third is the final chapter where Lena, Bunch, and the baby head to Tennessee in search of Joe Brown.
Now, these three endings give us three different perspectives on the individual's relationship to society. Christmas's death seems inevitable – a final, physical manifestation of decades of social alienation, anger and loneliness. Christmas learns no real lessons about himself or his racial identity, nor does he ever forge a meaningful connection with anyone.
In Hightower's death, we see the value of reflection and of being really honest with yourself. He finally realizes that his wife's death was his fault, that he was kind of a bad minister because of how obsessed he was with the Civil War, and that he was a selfish jerk a lot of the time. It takes a lot to admit these kinds of things to yourself, but Hightower does it through pain-staking reflection and thought, which eventually leads to a kind of en-"lighten"-ment that nicely coincides with the novel's title. Ironically, this enlightenment comes way too late for either Hightower or the world to benefit from it, and he dies almost immediately after having come to these revelations.
So where do we turn to for some hope and a little uplifting? Well, in the strange threesome of Lena, Byron, and the baby we see a modern version of the American family, tied together by ethical choices and a sense of responsibility (rather than marriage). Instead of running away from responsibility either physically (as Christmas so often did) or mentally (as Hightower did), Byron Bunch retains his position as the moral compass of the novel, choosing to stand by Lena even if that means helping her find the father of her child. The novel seems to suggest that honoring social ties – however tempting it is to abandon them – is perhaps the surest way to enlightenment and happiness.