Why do we draw the line at chemical weapons? (Details)

I know they are terrible. War is terrible. Why is gas worse than burning, shooting, chopping people up etc?

I’m upset about the gas attack, like everyone, but over 100,000 people already have been killed in the Syrian war. Millions have been displaced. Who knows what atrocities will be uncovered, when the dust settles.

Shouldn’t those who are upset about the gas, have been upset a long time ago?