Also, I can think of two very simple reasons why Assad would use chemical weapons despite the risks and even while he seems to be winning. This seems to be something some people are having problems with.
First is that he's been using them for ages and this is just the first time he's been caught. I read an interview with a Syrian in the affected area recently saying how they actually were used to chemical attack and recognised the chem canisters on the plane coming in, but they expected chlorine gas rather than sarin.
Second is that this is entirely calculated and aimed at the morale of the rebels. Basically him demonstrating that no matter what he does, nobody is going to help them. He's saying 'the whole world condemns the use of chemical weapons, but the Russians are on my side and the US don't want to get in another ground war in the middle east and that means nobody is going to meaningfully interfere with me no matter what I do to you'.
FWIW, I think it's a mistake to paint Assad as 100% Putin's puppet. Seeing Russian 'control' everywhere in the wake of the US election is a mistake. I suspect Assad did this off his own bat, and the Russians found out after the fact. Now the Russians are dealing with the fallout by muddying the waters and sowing doubt about the real events, cos Putin doesn't really give a ■■■■ about dead people but he does care about keeping his ally intact cos a big part of his mystique is that of the infallible mastermind who orchestrates everything flawlessly. Putin doesn't mind looking brutal, Machiavellian, or tyrannical, but he minds very much if he starts looking weak or foolish. His brand is based on a perception of strength, ergo his allies.must win (despite their flaws) because if they don't, the value of his aid is called into question. I suspect Assad knows this and has, on occasion, been able to exploit it. He and Putin aren't mates, but they find each other useful.