Use of Weapons - Iain M Banks (Book 3 of the Culture series)

The man known as Zakalwe was one of Special Circumstances' foremost agents, changing the destiny of planets to suit the Culture through intrigue, dirty tricks or military action. Though the woman known as Diziet Sma had plucked him from obscurity and aided his stratospheric rise, she did not know him as well as she thought. The drone Ska­ffen-Amtiskaw thought it knew both of these people.

It had once saved the woman's life by massacring her attackers, and it believed the man to be a burnt-out wreck - but not even its superlative machine-intelligence could see the horrors in his past.