There appears to be much evidence that it was in fact the rebels that used the chemical weapons which were supplied by the Saudis
Your "source" is a bunch of TV broadcasts by Syrian TV, which is controlled by the Syrian government. That's not actually a news outlet, it's just the propaganda wing of the Assad regime. They're going to say whatever they need to say to keep Assad in power.
The thing to keep in mind here is the scale of these attacks. You could see the rebels launching an attack on a small scale, maybe. But the attacks were actually launched against three different areas near Damascus, and used rockets and chemical weapons, and killed something like 1400 people and injured more. This kind of large, coordinated attack takes a large, well-trained, well-organized military with heavy weapons in the form of rockets and large chemical weapon stockpiles. The rebels are a bunch of disorganized factions with assault rifles; they just don't have the weapons or organizational ability to do that.
As for why would Assad use chemical weapons? Well, why wouldn't he? He'd previously sent in tanks and soldiers to put down demonstrations. He'd sent paramilitary groups in to murder civilians in their beds. He'd used artillery against civilians. He'd previously used chemical weapons on a small scale. He'd killed 100,000 people. All the while, the U.S. had done nothing, and Putin stood up for him and was going to veto any UN resolution. Assad had already gotten away with so many atrocities, he drew the conclusion that he'd get away with this one as well. And so far, no one's proven him wrong...