Italy used mustard gas against Ethiopia at the start of WW2. Those stores of mustard gas were mined when Rommel pulled out of Africa after the second battle of el Alamein.
After WW2, the Germans were brought back to the Qattara Depression to fence it off. Today, armed U.S. drones still monitor 24/7 those stockpiles near Benghazi.
It is quite likely that Ambassador Stevens was attempting to give that Libyan mustard gas to the Syrian rebels so that Assad would instead get blamed for using it inside Syria.
One thing I never see discussed is what will happen when the muzzies gain control of the chem weapons if the Assad regime collapses.