We know that Johnny and Roth "lied" to Fredo about something. Perhaps Fredo DIDN'T know it was going to be a hit and Ola told him that the men were just there to "snoop" around a bit. Or..Fredo DID know it was going to be a hit, and Roth/Ola didn't "lie" to Fredo per se, but they DID give him strict assurances that Mike would be DEAD. When Mike lived, Fredo crapped his pants b/c he knew that his kid brother would eventually find the traitor.
On this point I think it is justifiable to refer to the second draft of the GFII screenplay, where we see that Fredo thought it was going to be a kidnapping, not a murder, and that this is what Ola lied about.
