I think Michael may have been ready to give Fredo a pass, but his outburst in the boat house sealed his fate. Fredo not only wasn't repentent, but he tried to justify betraying Michael by admitting he was jealous and resentful; admitted he betrayed him for personal gain ("there was something in it for me"), admitted he witheld valuable info from his brother (Frankie's survival, Questadt belonging to Roth). Hard to not think of Fredo as not only deserving of death, but a threat if kept alive.

As for Geary: I've always believed that Geary helped set Michael up for the perjury rap. As a member of the subcommittee, he had to know that Pentangeli was alive and would be produced as a witness. I believe that, when he asked Cicci if there was always a buffer between him and Michael, he knew Cicci would say, "No, I never talked to him." That would make Michael think that could perjure himself because no one could back up the subcommittee's charges against him.