If Ola had said to Fredo: "We need your help in killing your brother," Fredo would have said no. But Ola probably couched it the way Fredo told Michael: Roth needed help in closing the "big deal." Where Fredo dissembled, IMO, was when he told Michael that Ola said, "there'd be something in it for me." Fredo would have wanted to know what was in it for him; and if Ola had told him that he'd get the Donship he deserved, Fredo would have inferred that there was only one way he could get it. And, even if Ola didn't explicitly promise him the Donship, he'd figure out why Ola wanted him to open the drapes.

So, I believe that when Fredo told Michael, "I swear to God I didn't know it was gonna be a hit," the truth was, "Johnny Ola never said it was gonna be a hit." He let his greed for what he thought he deserved close his eyes, ears and mind to what would inevitably happen as a result of his "help."