Yeah, did consider that....... and yes, the Russians apparently used their S-400's also.
BUT , the anti-aircraft missiles are locked on by radar, and there is a difference in the radar reflection of a streaking missile and a large slower moving aircraft. I cant say with 100% certainty you're wrong, mate, but my feeling is that it was hit by a missile.
And the other thing is: the Israelis and the French had no legal reason to launch missiles at Syria, so between them they are responsible for the loss of 14 Russian lives.
Hope the GRU is on their way to deliver a pollonium sandwitch to Macron and something similar to Netanyahu.
GZ