I'm very intrigued into the character of Fredo, I believe he did know it was gonna be a hit. Hes the only one that could have left the drapes open as said before by Turnbull. Plus he acts like he didn't but when confronted by Michael in the boat house we see Fredo get really upset for the first time. He was always very quiet and clumsy. But here we see Fredo getting angry and its no coincedence that its right after Michael says "Haven't I always taken care of you?" Fredo wanted to be in charge but never got the chance. Plus if he wasn't in on the hit like he said which I think is bull. Why then afterwards wouldn't he help out the Family and show his loyalty to the Family by telling Michael about how Roth was gonna set up Pantangelli? If some scum bags lied to me about killing my brother I think I would want to screw up their plans if I could! He didn't though, he knew if the Pantangelli plan went through it could hurt the Family. Pure and simple he was a traitor in every way.