Tone.

The tone of this novel is tough. I've thought of plenty of words, but none of them really encompass the entire novel. Despite this, I've found a word that come pretty close, and that word is regretful. The novel is mostly about people looking back at events in their past and trying to alter what happened so that they feel better about their actions. They try to save themselves by telling the story with some half truths in it.
O'Brien doesn't come out and truly set the tone until the very last sentence. "I realize it is as Tim trying to save Timmy's life with a story," (page 233). O'Brien tells us that he is trying to save his old self through the story that he is telling. Each chapter tells a different story with its unique tone, but the majority show an attempt to correct someones past.
This tone that O'Brien creates adds a timelessness to the novel. At some point in a person's life, they will want to redo something from their past. Even if it isn't as extreme as the novel's examples, the feeling of regret is one that everyone can relate to, no matter their time period.