Ultimately its just posturing by the west. They look impotent and have to be seen to be doing something. This chemical attack gives them the fig leaf of legitimacy they need to stage a limited strike.
The US is becoming increasingly frustrated by the intractability of the Syrian crisis. What they actually want is a continuation of a secular state in Syria - albeit a slightly less tyrannical one.
Democracy is a fine idea but doesn't help western strategic interests when the people of a country vote for an Islamist democracy - as they tend to do.
But as others have pointed out, while horrific - in the truest sense of the word - the Syrian chemical attack only represents a very minor escalation of the ongoing death toll.
Something has to be done about ending the war as a whole, a quick "punitive" strike solves nothing.
The only member of the Security Council with any sway over Assad are the Russians. They are the key to any negotiated cease fire and longer term settlement. Isolating them by ignoring their wishes serves no-ones interest in the long term.