One of the “responsibilities” of having chemical weapons (and nukes) is to be able to provide the security such that they are not stolen, and to provide controls, such that deployment is only through authorized channels.
Not having that infrastructure in place is what makes the holding government accountable, even if there is unauthorized use.
We know that the rebels are good at faking media reports of faked attacks. We know that they have no compunction in killing innocents to further their cause.
It's too early to hold either side accountable for this event.
All we know is that it would be profitable for the rebels to stage such an attack, and idiotic for the Syrian government to make such an attack.