Why is gassing Syrians in 2013 worse than gassing Iranians in 1983? Is gassing Kurds worse when Iraq does it in the 1980s than when Britain does it in the 1920s? Why do we hear so much about the Germans' use of chlorine and mustard gas in World War I and so little about the American and British use of phosgene in World War I? The Japanese gassed the Chinese during the Sino-Japanese War. Hafez Assad gassed Syrians in the 1980s, with the Reagan administration's wink and nod—better a Westernized dictator than those Muslim Brotherhood boys. What is it about gas—some gas—that makes us so queasy?
The Germans gassed Jews and Gypsies, the Croatians gassed Serbs. We killed our own prisoners with gas from the 1920s to the 1990s. Wikipedia suggests that the excessive twitching and drooling of the decedent makes the whole thing particularly icky for the viewers. I guess in comparison, electrocution is kind of pleasant, and the firing squad is a day at the park.
Perhaps our red line should have to do with the action and not the methodology. Just a thought.