Also, from a profile of Strelkov on the New Yorker blog:
I warned about Putin handing sophisticated weaponry to pumped-up nationalist thugs on Dilettante's Winterings back in May. It took no great powers of prophecy on my part and if I could see what was likely to happen then so could a "great statesman" like Putin. I put the ultimate responsibility for the tragedy on Vladimir Putin, the man who armed and incited these troglodytes.
My speculation based on what we know already: The separatists acquired at least one BUK missile launcher at the end of June. Basic training takes several weeks. They wanted to show the Ukrainian armed forces they could shoot down Ukrainian aircraft at any height. A few days ago they managed to hit a Ukrainian Antonov at 20,000 feet. The downing of the Malaysian airliner was a demonstration of rebel firepower that went horribly wrong.
PS: There are now allegations that Strelkov is spinning a conspiracy theory that whole thing was staged and the plane was full of bodies which were already dead. Someone should have told him about Occam's razor before he came up with an excuse like that.