It may not have happened until the sequel, but Michael Corleone ordering Fredo's assassination started with a key scene in 1972's original The Godfather movie. Killing Fredo in The Godfather Part II's ending is arguably the defining moment of Michael Corleone's entire story. With the Corleone family riding high under Michael's leadership, the gullible Fredo betrays his brother to Hyman Roth, providing information that the villain uses to orchestrate an attempted hit at Michael's home. Michael eventually identifies Fredo as the leak, and orders Al Neri to end the middle Corleone brother's life while fishing.

In fairness, Fredo was never completely aware of his role in betraying Michael. Fredo almost certainly knew he was working against his family's interests, but was blinded by the promise of a cut from Roth. Fredo did not, however, realize his subterfuge would result in Michael, Kay, and their children almost being shot. Regardless, Fredo chooses to hide his betrayal of Michael, and this deception - combined with the initial act of treachery - seals his fate. Strangely, however, it could be said that the wheels of Fredo's demise were in motion long beforehand.