We don't know for certain if the Gas! GAS! in Wilfred Owen's devastating poem was chlorine, but we do know that it can kill and maim in the way he described. But when his poem was written, chlorine had already begun to play a completely different, quietly heroic role, going on to save hundreds of millions of lives over the course of the 20th century.
Check out clawsoon's great, in-depth post "Chlorine probably saved your life today"