4 ideas
Montale broke away and became a policeman, while the other two stayed in crime. When Manu is murdered, Ugo returns from exile to avenge him, kills the mafioso he blames and is then gunned down by police, leaving Montale investigating what happened to both friends from inside the institution that killed one of them.
A policeman who comes from the immigrant districts he patrols tends to talk, know names and de-escalate rather than raid. The force reads that as weakness or divided loyalty, so it pushes him to the margins. The people with the most local knowledge end up with the least institutional power, and repression wins by default.
The book treats Marseille as successive layers of arrivals: Italians, Spaniards, Armenians, North Africans and gypsies, each once the despised newcomer. Seeing the city this way exposes nativist politics like the Front National as historical amnesia, since most 'native' Marseillais descend from the previous wave of outsiders.
The Mafia, far-right political movements and parts of the police do not act as separate forces. They trade favors, violence and protection, so an individual murder reaches upward into a web of mutual interest. Because of this, solving a crime through official channels is structurally blocked, and justice falls to individuals acting outside the rules, usually at great personal cost.