I am now assuming what happened is the rebels shot down the plane using an SA-11 missile(s).
But apparently that missile cannot on it's own differentiate between military and civilian aircraft - you need additional radar assets for that. And the rebels do not appear to have that kind of radar system operational.
So, my guess is that they saw the Malaysian 777 on the radar, assumed it was a Ukranian transport plane (since they probably thought civilian airline flights would not stray into a known war zone) and shot it down with a SA-11 missile.
So, it was probably a reckless, though unintentional, mistake.
All of this still begs the question...what the heck was a modern airliner doing flying near/over a known war zone?