Discuss the possibility that the Syrian government used chemical weapons in an attack and point to the evidence for it.
What you say is true.  However, it happened under Russia's watch and no other possible story has been put forth.  Look at the clues: Assad has used them in the past.  Who else has aircraft in the area, let alone the sarin gas?   What is the motivation for anyone using the gas?  Is is possible that there are factions in the Syrian military?  When it looks like sh*t and smells like sh*t, then, you know...