The culture was a dystopia.
The interesting and unique aspect of the series, is that Banks manages to write interesting stories about a utopia.
A decent chunk of the stories has the reader follow around Special Circumstances, which is effectively a sort of space CIA interfering in the affairs of other cultures. The entire plot of Player of Games, spoiler alert for people who haven't read it, is that both the protagonist of the story, as well as the reader by the narrator, have been mislead and used as a pawn to facilitate the overthrow of the government of another civilization which afterwards collapses into chaos.
To me you can straight up read most of the books as satire on say, a Fukuyama-esque America of the late 20th century rather than a futuristic utopia.
Well, better, put
A lot of people when confronted with these revelations/questions have an existentialist crisis. For some, the solution is to deny the Culture.
Long story short, the Culture is a true Utopia and some people just can't handle utopias
The earlier novels (I am going completely from memory here) "Use of Weapons" and "Consider Phlebis"
The humans are pets of the machines.
The machines are vicious, and righteous. They use extreme, and creative, violence when they feel it is in their interests.
In the later novels I feel Banks lost his way a bit, got too attached to the characters. In the later Culture novels the main characters are invulnerable, he does not let any of them die. Whereas in consider Phlebus (spoiler alert) Horza dies at the end and his whole race is genocided.