Among other causes were Saddam's attempts at genocide against the Kurds,[0] his attacks on US and British forces patrolling the no fly zone,[1] his defiance of UN regulations,[2] his redirection and delay of distributing aid to hurt his own people while blaming the international community,[3] his army of fascist goons running the country--to include his serial rapist sons,[4] the training and financing of international terrorist groups,[5] and his attacks on and threats to destroy Israel.[6]
The ideal of providing a democratic model for other authoritarian regimes, proximity to Iran, a need for strategic air bases to support anticipated regional conflicts, and oil politics all may have played some role as well.
Moreover, information declassified in 2015 showed that Iraq had stockpiles of missiles armed with sarin, and internal plans to halt production during sanctions while retaining the capital and facilities to resume production as soon as sanctions were lifted.
The US had a policy of regime change in Iraq as early as 1998, and the conflict should really be seen as one from 1991-2003. In the 2000 Presidential debates, even Al Gore said Saddam should have been removed earlier, and that he wanted to arm rebels to topple Saddam.[7]
It was flagged because Saddam promising an interrogator he wouldn't use chemical weapons to start WWIII doesn't change any of that, and is barely relevant to anything.
Granted, it's an extremely unpopular war. It's extremely unpopular because it was one of the most poorly prosecuted wars in history, partly due to de-bathification deteriorating civil government and partly due to it becoming a magnet for rising violent jihadism.
It's not an unpopular war because Saddam was some noble innocent, some scapegoat who wandered into the crosshairs of geopolitical forces outside of his control.
[0] https://www.hrw.org/reports/1993/iraqanfal/ANFALINT.htm
[1] http://www.nytimes.com/library/world/mideast/081399iraq-conf...
[2] https://georgewbush-whitehouse.archives.gov/infocus/iraq/dec...
[3] http://www.usip.org/events/iraq-sanctions-what-have-we-learn...
[4] https://www.theguardian.com/world/2003/jul/23/iraq.suzannego...
[5] http://www.cfr.org/iraq/terrorism-havens-iraq/p9513
[6] http://news.bbc.co.uk/2/hi/middle_east/956084.stm
[7] http://www.debates.org/index.php?page=october-11-2000-debate...