I think you need to dial down the cynicism a little. There's no way murdering 3K+ people was an "inside job". Even if we accept your cynical premise that the goal was to manufacture consent for a war, it could have been achieved with far less loss of life, or perhaps even none at all. So my vote is firmly on the side of incompetence rather than malice. As Reagan used to say, if those folks were any good, they wouldn't be working for the government. Or something to that effect.