Welcome, Vince! There have probably been more threads here on the shooting and Fredo's role than on any other subject. svsg gave an excellent summary.
I'm one of those who believes that Fredo opened the drapes. If so, I have a hard time believing that he didn't know that violence would follow. Why else would they want him to open the drapes? So they could peeping-Tom Kay getting ready for bed? Look at the violent resentment Fredo expresses to Michael in that boathouse scene and it's not hard to conclude that he could wish his brother dead.