0) He does not respect territorial integrity of neighbour states
1) He houses international wanted terrorists, makes it possible for them to hide
2) He commits crimes against humanity (unjust prison and death sentences, violent crimes against political opponents)
3) He is ignoring UN Resolutions
Saddam Hussein was guilty of all of the above, that said:
0) There was no direct connection between Hussein and 9/11
1) There were no WMDs found
Three questions:
Should the United States (and the other opposing parties in the Gulf War) have left him in power or removed him then? Maybe they should have removed him then
Was the Iraq war based on false information, false reasoning and some deception? I would say yes it was
Was it still "just" to remove Hussein? That's the 1 million dollar question, and I am not 100% certain how I should answer...
[And to clarify: I am no fan of George W. Bush, indeed no fan of the Bush family, I do think the Iraq war was wrong, but I am always open to other arguments. Christopher Hitchens made an interesting pro-Iraq war case, you should look some of his articles from that time up]