If you read up on Iraq's history of WMDs, the relevance of Iraq's chemical and biological weapons was that Saddam's regime had already a long history of developing and using these types of weapons both against neighbors and its own civilians. When Saddam decided to invade and annex Kuwait, half the world united to act, drive him out, and eliminate Iraq's WMD programmes. After the first gulf war, the UN was in charge of verifying that Saddam's regime destroyed it's existing stockpile and WMD programmes, but Saddam not only actively prevented the UN from doing any form of verificarion but also outright antagonized the UN.

It was with this backdrop that the "Iraq has WMDs" campaign managed to get traction. If you learn history and pay attention to the events, you'll quickly understand that Saddam's antagonism and mockery of the whole UN institution, specially when they self-isolated, was an easy sell even with weak evidence.

Making this out to be a simple matter exclusively and bounded to the existence of WMDs is naive and outright ignorant.