I would like to believe that Fredo was just stupid, not a traitor. But I can’t. Just consider:

--He betrayed Michael for personal gain: “Johnny Ola said there’d be something in it for me.”
--His outburst at Michael showed he deeply resented being passed over for the Donship. That tells me either Ola implied, or Fredo inferred, that what was in it for Fredo was that he’d be the Don. Only way he could have been the Don was if Michael was dead.
--Fredo knew that Pentangeli had survived and the Feds were holding him, and that the Senate lawyer, Questad, “belonged to Roth.” Only way Fredo could have known that is if Ola and/or Roth told him. That shows he was in with them far deeper than he let on.