Anything's possible, I suppose. But the rebels and/or Russians are the only ones to have shot down any aircraft in this episode so far (so far as I know). And if the goal is to broaden the conflict, a Malaysian Airlines flight is a pretty odd choice when there were numerous other flights operated by big western airlines (British Airways, Lufthansa, etc.) also flying in the area.

There's also the fact that Ukranian officials have specifically said that Russian-backed rebels shot the plane down. We shouldn't accept those statements uncritically, but they're out there and they are worthy of some consideration. Meanwhile, I'm not aware of anyone credible, who is in a position to know, asserting that Ukraine is behind this.