I've always believed, as SB said, that Michael might have considered giving Fredo a pass until that wild, resentful outburst in the boathouse convinced him that Fredo would be a danger to him all his life. What's more, Fredo revealed that he knew Pentangeli had survived--and that the Senate lawyer, Questadt, belonged to Roth. How'd he know? Roth told him. Had Fredo volunteered that info to Michael earlier, he could have saved his brother a great deal of worry--and maybe reinstated himself in Michael's good graces.