I think a lot of the Bush folks thought of Saddam as a kind of rogue asset. They'd funded him to attack the Iranians, and then he went off reservation when he invaded Kuwait, and later tried to assassinate HW Bush. They knew he had chemical weapons (because they had sold them to him), and tricked themselves into believing he was developing nukes. 9/11 ramped up the paranoia and gave the Bush administration license to lie their way into war.