"In such high populated area, it should be hundreds or thousands of witnesses. But there is no one."
There have been witnesses who reported the presence of the BUK system in the area at the time. You choose to ignore them. There are pictures of BUKs with their numbers painted over in both Russia and eastern Ukraine. Why would the rebels and/or the Russians hide the identity of such military equipment? There are fighters on both sides who have killed civilians. Why would large numbers of civilians report what they have seen, knowing the wrong story could get them killed by either side?