What I had in mind with Fredo's taking in some of Roth's men was that Fredo had probably been led to believe that sharing forces, so to speak, with Roth although in a regular capacity rather than as assassins after MC might, in his weak mind, seem "good for the family" and also as if he was more involved with things, i.e. Roth.
No, I wouldn't give him credit for killing anybody, certainly not setting his brother up.
It would have been the "recent recruits" who opened the drapes for the assassins. Fredo wouldn't even have done that IMO.
