My suspicion is that Obama would love an excuse not to get the US involved in Syria for the same reasons that he has resisted calls to intervene for the last two years. I think his caution has been and is wise, because there are lot of downsides and unknowns. If Obama really wanted to act, after all, he wouldn't have handed veto-power for the decision to the GOP-led House of Representatives (which since 2010 has done nothing but obstruct his every move). If the House votes "no" and we don't move on Syria, my hope is that Obama did a good acting job; that it appears to Assad that we came really close to deciding to attack. That alone might make him change his calculus, realizing that the strategic benefits of using chemical weapons aren't worth the potential costs of American intervention.
After this recent and brazen use of chemical weapons by Assad, it remains to be seen whether the US will intervene this time. My guess is that congress will block the authorization. In either case, what if Assad continues to use chemical weapons? Many people are asking why killing with chemical weapons is any different than killing with machetes, bullets, or bombs.
Chemical weapons have been used only a few times in military history, and they don't seem to have been all that much of a game-changer. Chemical weapons were most famously used in World War I by Germany first, and then the rest. It does not seem like they made a major difference. The battles in which chemical weapons were used took place in wide open spaces, where artillery, tanks, and rifles can work well, and where wind can dissipate the gas to some extent. The gas did get into the trenches, but the infantry learned to put on gas masks and keep fighting. Chemical weapons were never able to break the stalemate of trench warfare.
Hitler didn't use chemical weapons in WW2 against military targets. This might have been out of a sense of personal antipathy towards them (he was gassed himself in WW1), but also possibly because he didn't think they would add much to the highly mobile German war strategy. The chemical weapons we sold to Saddam Hussein (repeat this five times silently before proceeding) were used against Iran in the 1980s, but they didn't break the stalemate of that war either. Saddam used his weapons on the Kurds at Halabja, but he wasn't contesting the city from a military perspective. Later, Saddam didn't use his chemical weapons against invading American units in 2003. I think he correctly assumed that they wouldn't slow down the US military, but they would harden its resolve.
Contrast the rural battlefields of France to urban Syria. Rebel snipers take refuge in hollowed-out buildings, rubble protects soldier and civilian from sight and projectile. Basements and strong buildings provide hiding spots and safe havens for Assad's enemies, and for families of civilians. Artillery, tanks, and jets can pound cities to rubble, and yet the enemy is not easily dislodged. In fact, such destruction can make it more difficult to wage urban warfare. Look at pictures of Stalingrad in WW2 to understand why the Germans could never eliminate all of the pockets of Soviet resistance.
Chemical weapons have never been used in an urban warfare setting that I know of, but they seem particularly well-suited for it and could be a strategic game-changer. Pockets of stubborn rebel resistance can be defeated with gas attacks rather than costly (for Assad's army) house-to-house fighting. Gas will find its way into the rebels' positions and hideouts, which will be in heavily-populated areas. Most rebels aren't going to have gas masks, and the civilians definitely won't. Syria has become so sectarian that Assad won't have to worry much about collateral damage. Gas used against Sunni rebels will only be killing Sunni civilians. Even the infrastructure of Assad's cities can be kept conveniently intact with the use of chemical weapons.
I don't buy the arguments that chemical weapons are no different than other forms of killing, which have already taken the lives of 100,000 Syrians. Chemical weapons are different. If Assad starts using chemical weapons with abandon in Syria, casualties may well indeed rack up at a pace that we have not yet seen. Those casualties will be disproportionately civilian. Meanwhile, every other dictator in the world who may face the prospect of rebellion in the future will take notes. Stockpiles of chemical agents elsewhere will grow, controlled and sold by thugs confidently standing behind the prevailing notion of the present that the international community has no legitimacy to act against barbarism. These weapons, much easier to make and much harder to track than nuclear weapons, would inevitably fall into the hands of non-state actors. If we don't deal with this problem now, we may have to deal with it later.