It was public knowledge that Iraq had used chemical weapons in their war against Iran. Maybe they didn't have any *left*, but they definitely had some at that point.
"According to reports from the previous UN inspection agency, UNSCOM, Iraq produced 600 metric tons of chemical agents, including mustard gas, VX and sarin, and nearly 25,000 rockets and 15,000 artillery shells, with chemical agents, that are still unaccounted for. In fact, in 1995, Iraq told the United Nations that it had produced at least 30,000 liters of biological agents, including anthrax and other toxins it could put on missiles, but that all of it had been destroyed."
"Reacting to Western criticism in April 1990 Saddam threatened to annihilate half of Israel with chemical weapons if it moved against Iraq."
So I guess my point is, they had demonstrated ample willingness to use them in the past.