That's an interesting theory, MMF, and one that I don't think has been suggested before.

The problem, though, is that Fredo was just too weak, IMO, to take on and kill two professional hitmen. With a gun, maybe, but I believe they were garroted.

The best theory, as bad as it is I believe, is that it was Rocco, although there is absolutely no evidence whatsoever to suggest that he was in any way dis-loyal to Michael. But perhaps, as Turnbull has suggested in the past, he was jealous over Neri's ascension and his own diminshed role in the family, although I'm not sure what he would have accomplished by helping to get Michael killed.

Rocco as the traitor could also account for his willingness to take on the Roth suicide mission. He realizes the eror of his ways, and seeks some form of redemption.

But the bottom line for me is that if it were Rocco, Mike would have figured that out and Rocco would have been long gone.

That's what makes the whole question a major plot hole. The only one identified as a traitor is Fredo, but his physical weakness, in my mind, makes it impossible to believe that he killed two professional hitmen.