As the careful reader has no doubt gathered already, I am not in favor of U.S. involvement in Syria. There is another side to the argument that I need to recognize.
Poison gas as a weapon of war, a so-called weapon of mass destruction or WMD, is very ugly stuff. I do understand, and have sympathy for, the viewpoint that whoever uses weaponized gas should be punished.
It is no accident that after experiencing the horrors of gas warfare during World War I, nations everywhere banned its use. As ugly as things got during World War II, no nation used poison gas on the battlefield. Decades later Iraq's Saddam Hussein used poison gas on some of his own rebellious people, still inexcusable.
Chemical, biological, and radiological weapons are considered heinous precisely because their aim is directed at civilian populations. Their purpose is causing as much misery and death as possible. Their goal is to accomplish, within enemy territory, localized genocide.