Look to Winward by Iain M. Banks

I read this book from The Culture series second (after Consider Phlebas) since it deals with the aftermath of the Idiran War (700 years later) and I wanted to see what happened, since I like Hamza in the first book and it was written as an adventure novel.  Well, this volume is definitely not an adventure novel.  It is very slow and much happens inside the heads of characters, with extensive thinkings, flashbacks and memories. At it's core it could be called a spy novel in the future as it is about a saboteur from the planet Chel, who wants to destroy an Orbital (Halo Ring) and kill 5 billion Culture citizens to get a revenge because of the deaths in the Chelgrian Civil War, which was (somewhat) initiated by The Culture meddling. 

Chelgrians are three-legged feline predators who have a very conservative, caste-based and religion-based society, which is not yet at the level of The Culture technologically.  Quil is a former soldier who lost his wife in the Civil War and doesn't want to live anymore, so he is recruited by a faction of Chegrians who want to kill Culture citizens as a revenge.  Quil is supposed to create wormhole in the hub of the Orbital Massaq and kill the Mind (very advanced, sentient AI with full citizenship rights) which will cause a part of the Orbital to desintegrate and kill billions.  Quil will also die, but that's what he wants, anyway.

Chelgrians have devices called "Soulkeepers" which save the full contents of their minds at any given point, so they can be reconstructed in an artificial environment, and thus can live forever, in a way. However, Quil's wife's Soulkeeper was damaged so she suffered a permadeath, and that's what Quil wants as well. The Chelgrian extreme faction meet with 'allies' who are actually renegade Minds of the culture, at an "Airsphere" - ancient worlds created by a long gone civilization and populated by titanic fauna like the behemethosaurs, which are hundreds of kilometers long and live for tens of millions of years.  Chelgrians receive the wormhole weapon from the 'allies' and go to the Massaq Orbital under a guise of wanting to persuade a fugitive Chelgrian composer to return to Chel. 

Everything seems to go undiscovered until the last 10 pages, happening during the long-awaited concert of the first brand new symphony by the Chelgrian composer since he left Chel.  The Mind of the Orbital tells Quil that he knew since the very beginning that he was there to kill him and destroy the orbital, but he didn't do anything to stop Quil because the Mind wanted to die permanently.  The Mind says that he cannot live with the memories of the massacres and mass-murders that he committed during the Idiran war and everything he tried to forget didn't work, so permanent death is the only solution.  He and Quil die together a no-backup death, while the rest of the Orbital lives, and the Chelgrian faction that planned the massacre is murdered violently. 

The book is a meditation on permanent death and the meaning of a finite life, as opposed to infinite oblivion.