The likely collapse of the Assad regime, and the even more likely massacre of the Alawite minority makes me think that the rumors of chemical and biological weapons being moved out of storage are very true.  If the Assad regime finds themselves surrounded and out of options, they could use the threat of releasing the weapons to win safe passage out of Syria, or they might simply use the weapons to turn all of Damascus into a massive funeral pyre, deciding that taking their enemies with them in a Gotterdammerung style mass suicide, is preferable to giving themselves and their families over to rebels bent on revenge.  It doesn't take a lot of guys to open canisters of nerve gas or set off a few chemical warheads, and the rebels are certainly not prepared for such an assault, so it could get very ugly if the regime finally runs out of options.  If winning isn't an option anymore, and the Assad regime seems to be unwilling to negotiate a serious peace deal, then the MAD (Mutually Assured Destruction) option could become frighteningly real.

Unfortunately Syria is not like Egypt, where the powerful and independent military kept stability largely intact.  Syria will bleed long after Assad flys off to a luxurious exile.