Not that he destroyed them, but merely like it seems to be the case that they were lost/destroyed in the aftermath of the first gulf war.
Before the inspectors left they had no evidence that saddam had an active wmd program or any signs that he might pick it up any time soon.
The only evidence so far gathered were the tools stored in some scientist garden and according to him Saddam was waiting for the embargo to be lifted so he could start again.
Ok but the cease fire clearly stated he needed to destroy stockpiles of WMD and provide access to the UN inspectors. He failed on both accounts. They had multiple declarations of weapons programs and stockpiles through the 1990s after they got caught with their hand in the cookie jar. In 2002 I see no reason for anybody to believe Saddam didnt have WMD.
I suggest reading my link to get a better idea of what we were dealing with through the 1990s and the mindset politicians and people had in 2002. You cant apply today's knowledge to 2002 and say we were wrong in 2002. We obviously havent found anything in Iraq yet and our intel was bad on the WMD. That is my point, if we are proven false afterwards, then we are justified.