I don't see any way to justify it. The President hasn't officially noted he's against the Assad regime (far from it actually), so he doesn't have the grounds to strike them on that account. Moreover, in regards to the chemical weapons, yeah, they're bad, and yeah, Syria did accede to the Chemical Weapons Convention, but strictly speaking, the use of chemical weapons is only prohibited by that convention, not custom. Syria could leave the treaty at any time and thereby reserve the right to use chemical weapons (That they haven't confuses me to no end but that's another story) The CWC doesn't grant other countries enforcement powers should a member of the convention break their self imposed obligations. Thus, I don't see why the United States, the UK and France think they have the ability to strike Syrian military targets. Is it because those states are great powers and have the right to intervene in conflicts where and how they see fit? Is it because the situation is chaotic so no one cares if an American missile finds its way into the fray? Or is the United States now giving itself the precedent to destroy chemical weapons stockpiles wherever and whenever? I don't like any option.
In reality, chemical weapons are no more destructive and indiscriminate than a bomb. The hate towards chemical weapons is a bit irrational, especially when placed in context of certain types of conventional weapons. I guess there's an aversion to using poison to kill rather than mutilation and destruction. Less visceral, you know. If countries want to prevent civilian deaths, they'd do just as well and perhaps better to destroy Syrian aircraft and missiles.
Don't get me wrong, though, I would like to see chemical weapons gone from the world, if nothing else to make it harder to wage war. An irrational aversion can still be useful for arms control. Peace is the goal, after all. However, if the United States really wants to get rid of chemical weapons, it would be better off waiting until the war is done and then suing Syria in the International Court of Justice on the grounds that they broke the terms of the Chemical Weapons Convention. If such a suit were successful, it would strengthen international law and customary practice regarding such treaties. At the very least, it would call a spade a spade and force Syria to figure out what it wants to do about chemical weapons, either by renouncing weapons or just leaving the treaty. I certainly agree with the President that Syria should not be allowed to break the treaty and be allowed to get away with it. It's just right now the US is opening itself up for a suit from Syria against the use of American force without legal justification.