We DID talk about motive before in how Michael seemed to favor Neri over Rocco. Not a BIG selling point, but it IS there.

Roth could also have Fredo set up for the kidnapping and unbeknownst to Fredo, approach Rocco for the assassination (and subsequent killing of the assassins). After all, I really don't see enough time for Fredo to regroup his nerves and order his men to go kill the assassins (the whereabouts of whom he wouldn't really have a clue)

If anyone had the power, ability and knowledge to set up the hit and kill the assassins afterward, it would be Rocco.
But the motive IS pretty weak unless we could know what Roth had promised Rocco in exchange. The pot would have to be pretty sweet. And with Michael gone, it could have been.
After all, Rocco was a businessman.