Removing Assad at this point would leave either an opening for a permanent residency by the US or an open door for the Al Nusra Front to take over. Al Nusra, the Syrian branch of al queda was actually caught with sarin gas and appeared to be planning an attack with it. So I'm not even convinced it was Assad who did this.
It's really too late at this point as we've been funding the SFA for so long, much of those resources defected to Al Nusra with their "beheading of anyone against them" policy. So they've grown massive enough, they could likely take over even without our help. We already have a crap load of blood on our hands, so what's one more bankrupting permanent occupation to help support the military industrial complex, right?