This for me is the question that needs answered.
Around 12 hours after the bombing the inspectors were due. Surely if you want to prove to the world and give the world evidence you would not destroy a chemical facility.
Maybe the 3 nations involved were helping the Syrian people. What are the chances of Assad dropping another chemical attack on his own people again in those 12 hours with the OPCW due (highly unlikely, if indeed it was him with the original attack (Likely).
Maybe there is something or someone on the inside they were trying to protect?
But back to that question. Why destroy eveidence with inspectors due?