Something about gas is . . . creepy. Often, you can't see it. Supposedly, you can't run from it (not so, but it can be hard to tell which way to run). If it "gets" you, it doesn't leave visible marks, whether it kills you or not (again, effectively not true, but particularly on survivors, the outward signs disappear pretty quickly).
In this article in the National Review, the writer scrutinizes pictorial and other published evidence connected with the recent deaths of women, children, and old people in Syria, and his analysis is informative and persuasive. If scrutiny of this kind had been allowed during and immediately after the times when the Germans are said to have gassed millions of Jews during World War II, the stories about gas chambers would never have gotten off the ground.
But the victims in Syria were Arabs, so we can talk about it without fear of having our books banned and getting thrown into jail.
It makes all the difference in the world.