What's completely undisputed is that Assad's planes bombed this location. They deny even that (the absurdity of which should tell you all you need to know about whether they're telling the truth about not using sarin), but even Russia admits that they obviously did. Everyone saw and filmed the planes and no one but Assad has that air capability in the region.

Everyone also saw that they were bombing with gas, but set that aside because maybe the rebel groups on the ground are just lying about that.

While Syria is denying that they bombed anything at all, Russia is saying that they were bombing a rebel weapons manufacturing plant and that's where the gas came from.

But the rebels don't have the ability to produce sarin, sarin doesn't get spread like that if hit by munitions, and those same planes that Syria claims were never there in the first place came back hours later and bombed the hospital.

Taking this story as a whole, there's just no reasonable alternative for what might have happened.