Please excuse me if I try to clear my head on this, but I'm sure people here will correct my thinking. I have some experience of the ground side of Air Defence and also Int at MoD, so I'm not just an old ATCO.
The Exam questions in my mind are ... Who could have done this, and Why?
Russia.
Motive. They have not been attacked on their own soil, so no reason to target any aircraft.
Opportunity. All the kit you would ever need.
Rationale? Unknown, unless imagining some convoluted tin-foil hat plot.
Excuses? Very unlikely this was an erroneous engagement by whatever the PVO is called this week.
Ukraine.
Motive. Minimal, unless they did it and were trying to blacken the separatists.
Opportunity. Suitably equipped.
Rationale? None. Any idea that they engaged MH17 to discredit the rebels is ludicrous.
Excuses? Ineptitude has been mentioned. But a target at FL330 heading east is hardly a target, even if they had SAM systems in a suitable location with appropriate C3.
Separatists.
Motive. Apparently shoot at anything, air or ground, that potentially interferes with their desire to take over Eastern Ukraine.
Opportunity. Lack of "adult" C3, apart from assumed input from Russia. Systems acquired from takeover of Ukrainian bases, including Buk launchers (and associated vehicles?)
Rationale? Rational behaviour by the separatists (with or without Russian support/assistance) has not been in evidence since this started.
Excuses? Inept, hyper-active, lacking responsible leadership and command structure?
"I rest my case, My Lord"
(PS ... i'm not wasting time chasing bits of unverified information, or anything like that. I'm trying to get back to basics in my mind, and work from there)