Who is to say that the Ukrainian rebels haven't been armed by the Russians, or have the Russian military fighting with them?
Maybe they mistook it for another transport plane?
http://uk.reuters.com/article/2014/07/14/uk-ukraine-crisis-idUKKBN0FH09520140714
Photos of the scene. Some at the bottom of the page are graphic, avoid if you're squeamish:
http://www.buzzfeed.com/mikehayes/malaysian-airliner-crash-ukraine