As usual when this topic comes up, I can only add the following:

While I really don't have a theory which I'm crazy about (Turnbull's #2 or #3 are OK, I guess), I don't believe that it was Fredo's doing.

At that point, the best thing in the world for him would be to have Roth and Ola dead, taking what he thought was the secret of his betrayal with them. Why help try to keep either alive?