What do you need as proof?
"Why would Assad, knowing western attitude to chemical weapons, risk throwing away the gains he has recently achieved [?]" . Your defense is based on a person who is thinking rationally. Why be skeptical of how the gas was released (who did it) and yet trusting of Assad who DID use gas before? Did not the US recently say, it is up to the people of Syria to determine their fate?
You are right that death by conventional weapons is just as obscene. It seems odd that we don't have the same level of disgust. But we have international agreements that say, using gas is going too far in war. I actually am glad there is some limit.