I think Fredo may have bought into the lie that it was going to be a kidnapping and not a hit. He probably used every ounce of his own denial to believe that. But ultimately, he knew in his heart what he was doing with the motive that he would then become the don.
Except when does the movie ever say or even hint that Michael would be kidnapped? What would the purpose of that even be? Fredo would know exactly what he was doing when opening the drapes.
