The central tragedy of "The Godfather Part II" hinges on the devastating betrayal of Michael by his own brother, Fredo. Driven by resentment and a feeling of being disrespected, Fredo provides information to Hyman Roth's associate, Johnny Ola, which facilitates an assassination attempt on Michael at his Lake Tahoe home. Michael uncovers the betrayal in Cuba, when Fredo, drunk, lets slip that he knew Johnny Ola before. Michael confronts him with the chilling line, "I know it was you, Fredo," sealing his brother's fate.
After their mother's death, Michael appears to forgive Fredo, embracing him at the funeral. This, however, is a ruse. In the film's most heartbreaking sequence, Michael watches from his house as his enforcer, Al Neri, takes Fredo out fishing on the lake. As Fredo recites a "Hail Mary" to catch a fish, Neri shoots him in the back of the head. This act is Michael's point of no return, the ultimate sacrifice of family for power, and it leaves him completely isolated.
Simultaneously, the film reveals Kay did not have a miscarriage but an abortion, as she could not bear to bring another of Michael's sons into his world of crime and violence. This revelation shatters their marriage and solidifies Michael's isolation. The film's final moments intercut Michael's consolidation of power—having Roth and the Rosato brothers killed, and forcing Frank Pentangeli to commit suicide—with a flashback to a happy family gathering for Vito's birthday years earlier. The juxtaposition is crushing. As the family rushes to greet Vito, a young, idealistic Michael sits alone at the table, having just announced he joined the Marines. The film ends with a close-up on the present-day Michael, alone in his compound, his face a mask of hollow victory and profound solitude, forever haunted by the choices he made.