There is still no clear proof that Assad launch the chemical attack, I believe it was in fact the rebels who launched the attack. Why?
1) Assad is not stupid, he is a doctor who was trained in the UK, and he saw what happened when Iraq didn't let inspectors into his country. 2) Assad is winning the war, and the only way that he can lose is with Western involvement. 3) It all seems too convenient. The rebels were caught with 2kg of Sarin in Turkey. The attack happened to be in the part of the city controlled by the rebels.
The rebels need western help to win, and if killing 500 of their own people is the only way to do it, I have no problem with believing some Islamic fundamentalist would do it to gain power.