The Ukrainian rebels, who want to separate and join Russia, did it by mistake. They thought they were shooting down a Ukrainian government plane.
As for Obama, don't mind him. He's just an impotent spectator in this particular war.
"The Ukrainian rebels, who want to separate and join Russia, did it by mistake."
That's the most likely scenario. People who don't have any means of checking commercial flights vs military flights but do have enough knowledge to lock up a target then launch are probably the shooters.
Other things are possible, but nothing is any where near as probable as people who figure their SAM capability is a "use it or loose it" weapon downing the plane.
Nitwits with advanced weapons are a scourge to both their supporters and their enemies.
Just look at Hamas doing far more damage to their own people than to Israel. The Ukraine separatists are so far not quite as bad but with the increasing use of intense artillery fire by the Kiev side, they're going down the same path.