The people who should pay for the 9/11 attack are the people who planned and ordered it, bin Laden and al Qaeda. Saddam and Iraq had nothing to do with 9/11. Saddam and bin Laden were enemies. There were no al Qaeda camps under Saddam. There were two in the north of Iraq protected by the US "No Fly Zone."
Bin Laden and the al Qaeda camps were in Afghanistan and when the US invaded Bush allowed bin laden to escape by not pursuing him and instead attacking Iraq.
International cooperation and police work had made dents in al Qaeda's leadership and could have tracked him down and brought him to justice. Instead hatred of the US has grown around the world and so has al Qaeda and extermists becasue of the US crimes and atrocities in Iraq.
The real reason Bush attacked Iraq was to set up bases and control the region and the oil. In Cheney's energy task force, before 9/11, he met with oil coporation heads. He has refused to release most of the transcript but what was ordered released by the Supreme Court showed Iraqi oil fields.
Former Treasury Secretary Paul O'Neil has written that Bush and Cheney were looking for ways to attack Iraq when they first came to office and the "Project for a New American Century" of which Cheney and others were members had written in the 1990's of the importance and need to control the oil.