There has not been ONE reason given for Assad to have used these weapons

[muriel_volestrangler]

35. Juan Cole on why it might have been Assad's forces

Some have asked why the regime would risk using poison gas when it has been making gains against the rebels. But the regime’s advances are minor and tenuous. It only took the small town of Qusayr with Hizbullah help! And ‘advances’ in Homs were just scorched earth destruction of neighborhoods. They were offset by loss of a major air base near Aleppo, key for resupply of troops up there because roads north are insecure. The regime can only advance here or there, but doesn’t have manpower to take back substantial territory.

My guess is that rebels in Rif Dimashq in outskirts of the capital were making inroads toward Damascus itself. Defensive troops are off tied down in Homs. Since the capital is the real prize and end game, the regime decided to let them know it wouldn’t be allowed. It is the typical behavior of a weak regime facing superior demographic forces (the Alawites are far outnumbered by Sunnis) to deploy unconventional weaponry. Although there was a risk in using the gas, the regime may have felt threatened enough to take the risk, confident that it could muddy the waters afterwards with charges that it was actually the rebels who were behind it.

I don’t find the ‘false flag’ narrative about the gas attack put forward by the Russians plausible. Rebel forces are not disciplined enough to be sure of being able to plot and carry out a mass murder of the families that have been sheltering them in East and West Ghouta and to keep it secret. How could they have been sure no one among them would get cold feet and blow the whistle? Killing hundreds of women and children from your own clans would be objectionable to at least some in any group of fighters. The fighters in Rif Dimashq are not the hardened Jabhat al-Nusra types. Besides, capturing and deploying rocket systems tipped with poison gas is not so easy; even just operating them takes training.

http://www.juancole.com/2013/08/signals-intervention-syria.html

Note that he is sceptical about American intervention; but he does think it's more likely to have been the government forces than rebels.