You're assuming it had to be intentional, which is an overwhelmingly unjustified assumption. All it would take is the person in control of the weapon to think it's a military transport.

I think everyone is assuming it was an accident. Coming at it from the direction of trying to explain why anyone would choose to shoot down a passenger plane probably doesn't help clarify the situation.

Most likely, it's an accident. Somewhere else in this thread, there's links to a Russian separatist claiming to have shot down what he thought was a military transport with a Buk missile system that they bragged about having several weeks ago.

This seems to make the most sense. I can buy that maybe Russia slipped them parts of a high-end anti-aircraft missile system. Maybe enough to shoot it at an aircraft they saw. But it's plausible that they don't have the high-end air-search radar and/or the personnel with experience in managing the results of such a radar, or with the experience in operating it in cooperation with an anti-aircraft missile battery, that would allow them to reliably identify aircraft and direct attacks against only against the desired targets. They also likely have reason to be on alert to aircraft coming to attack them.

I would also think that the Ukrainian military is likely to have proper air-search radar and the experience using it to identify aircraft properly. And not much reason to be on high alert for potentially hostile aircraft, since the rebels don't have any, and Russia isn't in the fight on that level now.