“I am a man of honor,” Tommaso Buscetta insists. It’s a familiar enough assertion for a man in his profession — he describes himself as a “simple soldier” in the Sicilian Cosa Nostra — and one that Mafia-movie fans have heard many times before. The context this time is a little different, though, since Buscetta (who goes by the nickname Masino) invokes his honor to justify his betrayal of the organization he had loyally served for his whole adult life. No omerta for him.

Buscetta, the real-life subject of Marco Bellocchio’s tremendous and meticulous new film, turned on his former colleagues in the 1980s, testifying against them first in interviews with the Italian judge and prosecutor Giovanni Falcone and later in open court. “The Traitor,” anchored by Pierfrancesco Favino’s shrewd, subtle and volcanic lead performance, is less an exploration of the man’s motives than a chronicle of his actions. Bellocchio’s approach to the story is at once coolly objective — the movie is part biopic, part courtroom procedural — and almost feverishly intense. He has a historian’s analytical detachment, a novelist’s compassion for his characters and a citizen’s outrage at the cruelty and corruption that have festered in his country for so long.

Now 80, with a career stretching back to the mid-1960s, Bellocchio has long been preoccupied with the endless, tumultuous, tragicomic question of Italian identity. The dialectic of loyalty and betrayal — within families, local communities, political movements, subcultures and the nation itself — is his great theme, and Masino Buscetta supplies him an aptly contradictory hero.