If a leader used gas to kill, why would he be such a coward to deny it? I am not sure he would use it. In the long run he stands to lose much more than he gains by making enemies of the world..
Imagine admitting to gassing thousands of your own or cleansing your own. World's most wanted topper.
They do it and all blame who ever they are gassing or a new entry to the picture, not the original rebels but now Islamic State who are a law unto themselves did it, AKA some of the rebels who want take it up a notch.