And yet, unless we concoct a second traitor, there is no one else but Fredo. The only other possibility is a third man, who shot the two would-be assassins and then fled.

But Fredo, whatever his emotional and intellectual deficiencies, had cut a deal with Roth and Ola, and had allowed their hitmen to pierce the Nevada estate defenses. Roth and Ola must have deceived Fredo as to their true intentions, but he certainly had the wits to help the assassins to get close to Michael, so is it that far fetched that he killed them?

This wouldn't be like Fredo's failing to help Vito during that hit. In that case, Fredo was taken completely by surprise, and clearly did not have the nerve to fend off such an attack. But this would be much more premeditated. Fredo would be waiting by the culvert, perhaps a friendly face to the hitmen as they fled. He would have had the time to get his nerve up, and besides, what choice did he have? If either of the assassins had been captured, he would have been quickly identified as the traitor.

If Fredo was viewed by Roth and Ola as a good enough tool to get at Michael, then maybe he was a good enough tool to get rid of the hitmen.