I've just finished editing a novel of mine that's gone out of print (editing in this case is almost like writing the story anew; I've learned more about the craft, and markets and readers have changed). It's something of a mystery, a family seeking justice in the death of a child. There's an underlying concept to the novel – abused power disrupts the rule of law, and justice vanishes.
A last chapter is the place to sum up, to connect the story's specifics to the grander idea involved. As such, last chapters are difficult to write, and I had to spend several days on this one. You can't be preachy; you can't bludgeon the reader with your case-specific ideas. Still, your story has to make the end result inevitable as it exemplifies the concept you've spent so much time writing about.
Too, there must be a little distance between the story's inevitable end and the narrator's voice. The whole project of literature is to present such cases and their ends, all the while leaving it to the reader to plumb his or her own life experiences and to connect those to the story and its end.
It may be a coincidence that Troy Davis' case in Georgia, and his execution, have been on my mind as I wrote my story's summation. But it's cases such as his that create stories like mine, as we writers try, through the vehicle of fiction, to affirm life by assaying right and wrong.
I'll have more to say about the novel in a few months, hopefully as I'm able to have on its way to being republished.