Why would Bashar al-Assad use chemical weapons in defiance of clear threats by President Obama to intervene if he did so?
Dictators are often prone to taking risks: this is because they are generally invulnerable at home because of their security measures and repression. They are also surrounded by yes men who give them bad advice by anticipating what they think the dictator wants to hear and then saying it. Because of the dictator's paranoid tendencies in some cases, this is a natural survival strategy on the part of his advisers.
Dictators often believe that fortune or destiny favors them: Hitler used to talk about providence (his term for God--although definitely not the judeo-christian God). Hitler survived the Munich Putsch, successfully occupied the German Rhineland in 1936, pulled off the anschluss with Austria in March 1938, was successful at intimidating the Western allies at the Munich Conference in September, and a year later his army ran over Poland and then Norway, Belgium, the Netherlands, and France, and then his luck began to run out on him. But when he survived a serious assassination attempt in July 1944 he believed that destiny once again favored him. Hitler lost World War II in large measure because like Napoleon he did not know when to stop.
Saddam Hussein suffered from this same problem. After having entered politics as a young gunman for the Ba'ath Party in the late 1950s he survived a botched assassination attempt against the reigning strong man. After exile in Cairo in the 1960s he came to power with his cousin in 1968. He quickly established himself as the strong man behind the regime and eleven years later sent his cousin into retirement. A year later he launched an invasion of neighboring Iran. After eight years of expensive bloodshed a ceasefire was finally negotiated with no clear winner. Two years later he foolishly launched another invasion against a neighbor. This resulted in a quick conquest of Kuwait followed by a drubbing from a western coalition six months later after he stubbornly refused to withdraw in the face of a Western military build up in Saudi Arabia. He then survived a rising by the Shia majority in the southern part of Iraq. He squashed the rebellion and concluded that as long as he did not threaten Western oil sources he was safe to do what he wanted to his own population. Twelve years later suffering from sanctions but still fully in power he defied the United Nations IAEA inspectors in the belief that he was immune. George W. Bush, whose father he tried to assassinate a few years before, invaded Iraq, overthrew his regime, and eventually Saddam was found cowering in a hole in the ground.
Obama had repeatedly demonstrated his indifference to high civilian casualties in Syria's civil war. With the United States military still in the process of withdrawing from Afghanistan and the American population weary of two long wars in the Middle East, one of which ended inconclusively and the other which has yet to end, Obama had good reason not to intervene. But today in Washington there is a permanent infrastructure of liberal interventionists and neo-Conservatives who are continuously calling for American intervention either on humanitarian grounds or in order to look resolute. Obama set a red line of not allowing the use of chemical weapons by either side in Syria in order to appease his critics while keeping the United States out of the war. American interests have not changed. Why would Assad have risked such an attack? It could have been an unauthorized use by a subordinate, use by Assad to deliberately challenge Obama and thereby prove that he is a paper tiger, or even a case of faked evidence by the opposition.
Obama will probably respond to the chemical weapons usage in the suburbs of Damascus by launching a few cruise missiles at targets in Damascus as Clinton did to the attempted Ba'athi assassination plot against George H.W. Bush by launching cruise missiles against an intelligence headquarters in Baghdad. Clinton also launched cruise missiles against an alleged chemical weapons plant of the Islamist regime in Sudan in response to allegations that they were manufacturing chemical weapons. The missile strikes will probably have little real effect on the civil war, unless they are massive in scale. And Obama may fear doing such precisely because he does not know who will ultimately benefit from such a strike.
Assad will survive, Obama will claim that he showed Assad that there is a penalty to pay, and the pro-interventionists in Washington will claim that the weak response makes Obama look weak and feckless. It is all so predictable.