He was killed in a Russian airstrike!
Don't get fooled by the Russian propaganda. 90% of the Russian airstrikes in Syria are against the civilians and the rebels. Only the remaining 10% is against the Al Qaeda and the ISIS. The Russian aim is to support Assad, and not to fight the ISIS.
Well ofcourse they should be supporting Assad. Why wouldn't? This is pretty much the same situation, somewhat, as it was with Gaddafi and Algeria.