With that said, the Buk was also designed to surpass its predecessor('Kub') by incorporating a fire control radar into each launcher (TELAR vs TEL), so that it could operate with some autonomy. The original idea was to allow the whole SAM network to track multiple targets independent of the 9S18M1 radar, which would focus on detection and delegation.
This capability also enables an autonomous TELAR unit (as the various images and videos are showing) to target an aircraft at a range of 3-32km, much as people are hypothesizing happened to MH17. It is plausible, although far from certain.
I'm not 100% solid on this, but I have read that they were using just the missile truck without the supporting air-search radar. The missile trucks apparently have fire-control radar and can attack and aircraft by themselves. It seems plausible that they were given or stole just part of the system - the missile truck - and maybe got some manuals or some hasty training from Russian military or something. So they know enough to fire the missiles at a target, but don't have the hardware or training to operate a proper air-search radar and identify their targets carefully first.
Which is why I think it's mostly likely that it was them. Say what you will about the Russian and Ukrainian militaries, I doubt they'd accidentally shoot down a civilian airliner.