Based on what I have read (including from your fellow atheist Chris Hitchens) I will try to give the "pro-war" side of the story.
Saddam did not do 9/11 but he showed an unrepentant hostility for the west ever since the Gulf war truce happened (the Gulf war technically never ended until Saddam crawled away to his spider hole.)
He tried to murder Bush senior (GWB's daddy true, but still a former president of the US of A!),was a big sponsor of the second intifadah (Palestinaiins blowing themselves up at Jews) and repeatedly tried to shoot down American and British planes in the no fly zones. Coincidentally those no fly zones were the only thing stopping him from committing a massive genocide against Kurds or shiiites. Read sometime about how he repressed the 1991 revolts abasing him!
To those who say WMD were bunk that is only partially true. He apparently had a massive WMD stockpile and program through the 80s and up until the Gulf war including some nasty stuff!
https://en.wikipedia.org/wiki/Iraqi_biological_weapons_programThe main reason he did not have the weapons Bush said he did was because of the massive UN sanctions (unreplicated today) and the devastation following the first Gulf War. From what I understand, Hussein wanted to restart the program after the sanctions were lifted, and already had the scientists and blue prints ready to make them again.
To those who say Saddam had no connection to terrorism, you are sadly mistaken. Saddam played host to several major terrorists (including one of the original bombers of WTC in '93.) Look up Abu Nidal. He was the FBI's most wanted terrorist before Bin Laden came on the scene.
Saddam had extensive ties to terror groups, but Al-Qaeda was not one of them. It was mostly Shia extremists he disliked, and Sunni ones he was fine using and financing so long as they carried out his aims.
The whole "Secular Saddam" thing is a bit overplayed. Some say ISIS is just a natural outgrowth of his regime
https://www.nytimes.com/2015/12/23/opinion/how-saddam-hussein-gave-us-isis.html