Hell, I'd like to see the evidence it was Assad's forces that used them. If he was winning through conventional means, what purpose did it serve to do the one thing he knew would inflame world opinion? He's a thug but not a theocrat blinded by a deep desire to do Allah's will.
The "opposition" the other hand would love the propaganda windfall of gassing people and having the world blame it on the government. I mean, they are the same type of people that think nothing of strapping bombs to themselves or their kids and blowing themselves up in a market... Not too much of a stretch from there to gassing some civilians for the greater good...