Syrian President Bashar al-Assad surely knows that the use of chemical weapons is a red line his army cannot cross. Why even risk it? Why tempt the West to invade? Why tempt Israel to more openly aid Syrian rebels?
It's worth saying that we don't know for a fact that the Syrian Army has unleashed these weapons against the rebels, although House Intelligence Committee chairman Mike Rogers told CNN that there was a "high probability" that he had done so. How could the U.S. know this? My guess is that Israel's intelligence apparatus, which knows more about Syria's military that most of its commanders, has signals intelligence suggesting that weapons were used, and probably corroborated it with human sources on the ground. Just a guess. Israel knows where Syria keeps its stockpiles of vesicants, blood irritants, and nerve gases.
Syria has never confirmed that it has chemical munitions, but it has also insisted that any such weapons would be used against foreign invaders (Israel), not Syrians. So what gives? Chemical weapons appear to have been used near Aleppo at least once; the U.N. and U.S. say that the rebels aren't responsible. Put yourself in Assad's mind: His regime is in mortal danger. He believes that foreign invaders (Turkey, Qatar, Israel) are using the rebels as proxies. If he were to use chemical weapons to intimidate the rebels into surrender — limited use as a deterrent — then he'd be in a hell of a lot of trouble. But importantly, he'd be in trouble AS the still-sovereign leader of a country. (Saddam Hussein gassed the Kurds, and he was still treated as a world leader, until, of course, the U.S. decided much later to oust him). If the gambit failed — or fails, because it looks like we've reached a tipping point — then he might want to accept Jordan's offer of asylum.
Another possibility is that Assad does not have command and control of his army, and that rogue generals decided to use the munitions themselves because they're not cosseted in Damascus. Since Syria won't confirm it has chemical weapons, there is no public accounting of the decision chain used to employ them.
Assad's government says that the rebels used chemical weapons. Even if they possessed them, it would complicate an already complicated case for Western intervention.