The issue I personally see is that it makes no sense to use chemical weapons if you're winning "handsomely" already, especially with a huge arsenal of Russian made conventional weapons to do the job better than chlorine gas would.
Rationally I know it was probably Assad being not-smart, but using chemical weapons and invoking the ire of the rest of the world seems like far more of a threat to his position than the rebels ever were.
Again, I know it was probably Assad, but something stinks about it, it makes no logical sense for him to do it.