Last night I obnoxiously joined a group of people to watch a "9-11 was a big US government-run conspiracy" movie titled, The Great Conspiracy. I was obnoxious because I had little chance of believing the movie, and couldn't restrain myself from saying so afterwards.
I'd divide 9-11 conspiracy theories into 3 groups: 1. active conspiracy theories, like this movie, that say the US government or people therein were in charge of the attacks; 2. passive conspiracy theories that say that people in the US government knew Al Qaeda was going to cause 9-11 levels of destruction and deliberately chose to do nothing, and 3. negligent semi-conspiracies, where people in the US government knew they weren't doing enough to stop terrorism on US soil that could kill hundreds of Americans, but they either didn't care or thought it might benefit their right wing agenda. The categories can shade into each other to some extent.
The second and third categories are hard to prove or disprove. I personally believe the third category is quite possible.
The first category is a very different thing though, and I have two reasons for thinking it's wrong. First, none of the evidence seems very convincing, and many events do not fit the way I would have done things if I were in charge of a conspiracy to subvert American democracy and find a pretext to invade Iraq.
The second reason may be more convincing to me and less to others, but I do think there's a semi-limit to the evil that people in the US government will do. I say a semi-limit because there's no limit to the evil they'll cause, but if they can't hide the question from their conscience, "shall I unambiguously murder thousands of white, middle and upper class American citizens to advance my political agenda?" then they won't do it. Or at least there's not enough people who'd say "Sure!" to that question in the right places in government to pull it off.
Now using fake evidence to invade Iraq, causing the deaths of tens of thousands of Iraqis, and the death of 1,500 American soldiers (many of them white), now that's okay. Just like police are willing to use fake evidence to frame people they think are guilty. The rudimentary consciences of people in the Bush Administration doesn't stop evil from happening, but it does stop certain kinds of evil. Just my intuition of what they're like; your mileage may vary.