I still maintain that Fredo did not have to die.

Not only is his exact role in the hit unclear to us, it is also unclear to Michael. There's no indication that Michael ever learned what Fredo said or did to help Roth and Ola, only that he was somehow involved with them prior to the hit.

Also, the idea that Fredo was a continuing threat to Michael is belied by Michael's own actions. Fredo is hanging around the compound when Michael decides to talk to him. Even after that conversation - at which point Michael has clearly decided to kill him - Michael still allows him to come visit Mama. If Michael viewed Fredo as a continuing threat, there's no way he would have allowed Fredo to visit the compound.

Michael's lack of knowledge of the extent of Fredo's treachery, combined with his willingness to grant Fredo access to the compound, make it clear to me that Fredo's murder was simply vengeance, not self-preservation.

You might believe that vengeance is an appropriate motive for the murder of your child-like brother. I do not.