There's enough blame to go around here.
Blame the airline, or the pilot, or whomever was responsible for flying through that area. I don't care if other airlines do it (they're equally stupid), as a matter of simple common sense you should not be flying civilian airliners through a war zone, or a "hot" zone, or whatever the hip term the kids are calling it these days.
Blame the Russians, for destabilizing the region, supplying arms and military backing to the "rebels," and for basically thumbing their nose at the world's outcry. Had Russia not gotten all grabby over the Crimea, this plane lands safely at its destination.
Blame Vladimir Putin specifically, a modern-day wannabe dictator with more bravado than brains. Invading the Crimea was his idea.
Blame the insurgent leader who posted about it on VK. Maybe take a second to figure out what you actually shot down before going to the webz to boast about your great conquest, genius. Kinda takes away the difficulty of the investigation when you tell us you're the one who shot the plane down.