Why would Michael ask Kay why the drapes were open if they weren't opened by someone else in order for the assassins to have a clear view of the room? That line served as an explanation to the viewer that someone else had opened the drapes.
Exactly. I don't know why people go off on tangents and theories, making it more complicated then it is, when it's so simple and clearly presented in the film itself. It was obvious the drapes were kept closed, hence Kay's question. Somebody else had to have opened them, in order to give the hit men a clear line of sight. And that someone had to have access to Michael's home. And besides access, they had to have motive. The only one who fits is Fredo. Not Tom. Not Rocco. Not Neri. Not Connie. Not the ghost of Moe Green. Fredo. And that shows Fredo was lying when he swore to God he didn't know it would be a hit. That doesn't mean he didn't feel some sense of remorse anyway for the plot against his brother, in addition to fear of Michael and disappointment in his deal with Roth and Ola falling through. But he obviously knew Micheal being killed was a part of it. The end.
