What exactly were they targeting?
I think your error is to assume they were actually "targeting" anything. I'd suggest they were the same sort of untrained, undisciplined thugs who are to be seen spraying the countryside randomly with fire from their AK47s in "celebration" in third world conflicts everywhere.
New expensive toy supplied by Uncle Vlad. Minimal training, no discipline and no actual understanding of the world outside their immediate brain-washed surroundings. I doubt they were educated enough to realise that international flights take place over the little bit of disputed territory they call home. In their mind, they are fighting a "war" against a government that has aircraft. They don't have aircraft, therefore anything flying above them must be the enemy.
In the final analysis, the ultimate responsibility is with those that gave them these toys and encouraged them to use them. Unfortunately it is the way of the world that they are never held accountable.