Fredo was in deep with Roth, and had to go, period. He bent over backward to make it appear that he had never met Johnny Ola, even shaking his hand at the table at the gala as if being introduced for the first time. Only after he got too much liquor in him did he slip up at the peep show, talking about how Ola knew where all those places were, etc., revealing to Michael, who was listening nearby, that he was indeed connected to Roth and Ola, and was therefore involved in his assassination attempt. I don't buy that he didn't know it was "gonna be a hit" for one second.

Opinion question: if Fredo hadn't slipped up, would Michael have ever known of Fredo's connection to Roth, and the assassination attempt?