"I find even more risible the idea that the Russians shot it down. There is no motive."
There are only two logical motives:
1) the rebels shot it down thinking it was a military aircraft
2) some element within Ukraine shot it down deliberately to use as a modern version of the Lusitania
Taking the first as the simplest of those two logical options then the question becomes why they thought it was a military aircraft?
(Specifically an Il-76?)
Obviously there are a lot of possible illogical explanations as well for example the missile missed its original target and locked onto the airliner which is apparently what happened in 2001 with Siberian Airlines 1812.