Here's why there's a red line drawn at the use of chemical weapons. From Jeffrey Goldberg at the Atlantic:
Exposure to sarin gas, even in small doses, will cause victims to experience vomiting, diarrhea, an increased heart rate, eye pain and drooling. In larger amounts, the exposure can cause convulsions, paralysis, breathing difficulty and, within a short time, death. Sarin stops the body from controlling muscle and gland functions, according to the Centers for Disease Control and Prevention, and this causes muscles to twitch and breathing to become faster and irregular. Eventually, the body is overwhelmed, and breathing stops. On a single day in 1988, in a single town, Saddam’s forces killed some 5,000 Iraqi Kurds using a combination of mustard gas, sarin, VX and tabun.
Assad has committed many terrible crimes against his people, but if these latest reports are confirmed, he will have entered into the pantheon of the modern era’s worst war criminals, just as Saddam did in 1988. Back then, Saddam was considered an ally by the U.S. (he was in the midst of a war with Iran). So, to the everlasting shame of President Ronald Reagan, the U.S. did nothing to stop his genocide.