Russia's foreign and military policy since WW2 has always been to create buffers between them and any potential enemy. Since they lost control of the Eastern Block nations, this has become complicated. The grand strategy is to keep any neighboring country which can't be politically controlled, militarily impotent.
Both countries, US and russia had involvement in this proxy wars around the world. We did some of the same things in different countries.
So the two scenerios are
1) Ukraine wanted the rebels to shoot down a civilian airliner so they could gain popular support so they kept sending civilian airlines over and hope the Rebels would screw up and not identify the aircraft and shoot it down
2) Russia was helping the rebels and though giving them a BUK would help them, but somebody forgot to check what type of aircraft it was.