Fredo said, "The got Pentangeli, I can tell you that." Well, Michael already knew it because the committee chairman would have had to tell his lawyer, Tom, the identity of the witness to be used against Michael. But I have a suspicion that Fredo's next statement, "That Senate lawyer, Questadt, belongs to Roth," may have been a surprise to Michael. If so, it certainly implied that Fredo was closer to Roth and Ola than he'd let on--and that he'd had info that could have helped his brother if he'd come forward with it sooner. That alone may have cemented Michael's decision to whack Fredo.
If nothing else it confirmed in Michael's mind that Fredo was a dead man walking. It is proof that Fredo was not kept in the dark as much as he had let on, and that he was still lying to Michael, and possibly still trying to betray him so he could seize control of the Corleone family.
He keeps saying there was "something in it for me" and I think the something was being Don Fredo Corleone. Ola could have easily filled his head with this aspiration, suggesting to him that he had been passed over by his aging and foolish father, etc. etc. etc. Obviously with Michael out of the way and Fredo at the helm the Corleone family would be ripe for the picking by Roth and a few of Roth's allies who would take a piece just to make sure things ran smoothly.
