Some more considerations.
A few days ago the Ukrainian government and armed forces announced a non-flying zone regime above the rebel-controlled territories. This is because they were thinking that Russian military airplanes were flying there (actually, they were not). Could it happen that a Buk (SAM) military personnel pulled the trigger thinking that it was a russian plane breaking this warning? Why not, IMHO. Paranoia is in the red zone, everything may happen.
N.B. Buk systems (SA-17), as well as SAM-10/12 are regular arms of the Ukrainian army, none of the unit was smuggled by the Russians into the Ukraine territory to support the rebels. The rebels just do not need such systems. All their threats, such as attack Su-25s and helicopters fly low enough and could be shot with man-carried devices.
The rebels (again, they are the Ukrainians, not the Russians) could do it as well, theoretically. There were the rumors that they took some Buks from the army personnel who left their bases and dispersed. But this alternative seems less likely for me, because such systems are not easy to operate. Second, before running away from a military base and leaving such arms to separatists, the (Ukr.) army would definitely make them inoperable (just a matter of a hammer and 1 min to do the job). Third, the Ukrainian General Attorney has withdrawn statements (yesterday evening) that the rebels got Buks.
Statements here in this thread addressing the Russians to shoot the plane down from their territory are beyond the common sense. They could only do it with a huge C-300, which are easily traceable, visible, etc.. For me it sounds the same unlikely as if a USN ship would do it while in the Black Sea. And Russia is the least interested side in doing such a thing.
So, at the moment the option that seems the most likely to me (IMHO) is that the Ukrainian army did it erroneously.