If you let terrorists start using chemical weapons with impunity it's only a matter of time before they are used against US troops in the region or are smuggled and used in a big city.
ISIS and other rebel groups already have them, there are youtube videos of them testing the stuff out. I think they are more likely than Assad to use them against other countries but that is not who we bombed. It is my belief that terrorist will use whatever they can get regardless of what we do. Syrians killing Syrians is Syrian business. We should not get involved.