Let me see, Separatists with a weapon they figure is, "use it or loose it", thinking? Their preconception was that anything in the air is Ukraine military because Ukraine civilian flights have all been canceled (if they even thought that deeply), . . . .
Nope, they weren't thinking. The knew it was Ukraine military and no one was going to convince them otherwise even if anyone had tried. And who was going to try anyway? They all have the same preconceptions and are in a situation that rewards going with the what "everybody knows".
Just like whoever didn't warn civilian traffic to stay out of the area, they didn't waste any time thinking about civilian airliners being shot down.
And this is what I find most interesting --
The ICAO listed this area as a safe fly zone, an evaluation that the Malaysians et al trusted leading the airline into the crosshairs.
And then after the shootdown, Borodai and Putsin both came out and named that same ICAO as the organization that they would deal with and that they wanted to be in charge of the bodies, the boxes, and the investigation.
I suspect that the Malaysians didn't want the ICAO anywhere near this investigation while the Russians insisted that they must be -- and that is what held the bodies and box from being turned over for 4 days.