Youve very nicely presented a logical fallacy. Kudos to you. 

Of course we killed thousands. But we've followed international acceptable rules of war. 

Gassing innocent women and children violates accepted international rules of war. It is a war crime. 

And I love sharing quotes! Try this out for size. 
"The only thing necessary for the triumph of evil is for good men to do nothing." 

Also, you might not see any credible evidence, but I'll bet you dollars to donuts the intelligence community does. Hell, I bet you couldn't pop a zit on my fat ass in the ruins of a bombed out Syrian hospital without a satellite recording it live in high definition. Just saying.