As has been pointed out, this means he could continue killing his people by conventional means. Making it clear he is personally accountable for atrocities committed by his forces could avert this scenario.
The French text presented to the UN included a referral of the Syria situation to the International Criminal Court (ICC). How could this be sanctioned? The United Nations has twice referred countries that are not party to the Rome Statute to the ICC: Sudan in 2005 and Libya in 2011. The ICC prosecutor moved swiftly in those cases, getting approval for arrest warrants against Sudan's President Omar Al Bashir and Libya's Muammar Qaddafi. (I was the chief of staff for the ICC's president in 2009-11.)
Much evidence is being gathered of the crimes committed since the fighting began. Photos, videos and satellite images, used ordnance and personal testimony will help piece together the truth and prosecute those responsible. ICC referral is also only one option. Single jurisdiction courts, special tribunals or even a South-African style ‘truth and reconciliation' committee could be set up. But the only institution that can issue an arrest warrant now is the ICC.
But what difference would that make? It would surely lift the rebels' spirits if the world's highest independent judicial authority were to confirm that the man they are trying to topple is a wanted war criminal. It would highlight the immense suffering of Syrian civilians. It would send a clear signal that crimes against humanity are never acceptable. It would severely limit Assad's freedom of movement, since all 139 signatories to the Rome Statute would be under obligation to arrest him, given the opportunity. And it could prompt more of the Assad clique to jump ship, since warrants will be handed out for all involved.
Those opposing ICC referral point out that an ICC warrant would remove the option for Assad to save face by escaping into exile. But that has been the case for the last two years. Many saw the ICC referral of Libya as a warning shot to Assad, but he has taken no notice.
Self-referral by Syria is another option. Ivory Coast and Palestine have both referred the on-going violence in their territories to the Court. Since it is unlikely that Assad would allow his government to do so, this could only come from the opposition.
The Court would then face the tricky question of whether the opposition can be considered the legitimate representation of the Syrian people. A way forward could be if the UN General Assembly (UNGA), which unlike the Security Council is not a hostage to super-power vetoes, were to endorse the opposition. Again, the UNGA's recognition of Palestine could serve as a precedent.