As much as I don't like Assad (and his father was one of the worst criminals in the unfortunate history of Arab dictators,) I find it really hard to believe that he or his forces would use any type of chemical weapons during this conflict and especially at this stage. To do that, he's compromising everything and everyone that has kept him in power through these brutal 6 years. The Iranians, Hezbollah and most of all, Russia.
I don't even believe he was responsible for the previous chemical attack that resulted in all those US cruise missiles raining down on that airbase. Same story there. This is really the dumbest thing he could possibly do at this point and why would he do it? Makes absolutely no sense at all and this is pretty clear and smells of some type of set up.
If you look at Assad's doctrine, you might find the answer.
Assad uses the same tactic that Russia would use in a war. There has recently been a US handbook on how to deal with the Russian doctrine, which I read on War Is Boring someday past.
The main strategy is chaos. Barrel bombs and so much destruction isn't to kill people, but to shock them. It is the strategy of fear, to strike disorder by ensuring that some fighters are killed, some are helping civilians, and others are mourning for their dead families and friends. This breaks the, and when the people have had enough, they start to reject the Rebels, in hopes for the bombing to stop.
This can be seen clearly in the swiftness of the Jaysh Al Islam negotiating their relocation from Douma. There are already buses on the way.
Worst of all is the use of Chlorine gas. It isn't like Sarin. It sinks to the bottom, into the cellars and underground places, forcing the civilians out of their shelter. And there have been hundreds of chemical attacks, and each and every one of them save Khan Sheikhoun have been met with zero resistance.
Doesn't sound suspicious to me.
How many air forces that fly unabated over the skies in Syria are capable of dropping those things from such high altitudes?
Only Ru and SY planes can go into Damascus and suburbs, due to cover from S-400 systems.