[Figment]

C. Rebel on rebel action. It's no secret some rebels are fighting others for dominance. Unlikely since they are not in Assad terrain though.
D. Hezbollah. Not above killing civilians with bombings. Maybe they got their hands on them for the first time and just wanted to see what they could do with it.

Two other options.

Hard to tell, but it's not B1. You don't move firing installations like that around without getting noticed in Assad territory. Radar images from several nations were suggested to have seen the source of the missiles to be from Assad controlled terrain.

Additional suspicion upon Assad is raised by the following bombardment by Assad and the refusal to let inspectors visit the area for a long time. If it wasn't them, wouldn't they want to have it confirmed the rebels were using these weapons? Or is he just trying to protect the public secret that there are gas weapons in his hands?

The reason why? Rooting them out conventionally is a lot harder than letting Allah sort them out. I take it he's going to be quite conservative with his remaining own (human) resources (soldiers). After all, they're already within his own capital's city limits. And so far, conventional methods have proven unsuccesful in rooting them out. And without Russian and Iranian weapon supplies, what's he gonna fire with? Stones?

Assad's regime is not above torturing its own citizens for no reason. Wanton killing is not entirely out of character.