Seems to me the Ukrainian Su 27 hit the 777 with a missile, hence the U turn, realising it had gone to shit the Ukrainian military then used the BUK missile.
As in everything in war you have to ask who benefits, Qui bono, certainly not Russia or the rebels with the eyes of the world on them. The Ukrainians certainly do benefit, remember the background to all of this is GAS! More specifically Gazprom. This is why the tin pot junta was placed in power by the US in the first place, unelected I hasten to add. It was to break the back of the Russian gas supply to Europe, Ukraine has always been an important lynchpin in the geo political game with the US and Russia.
With the 777 shot down in rebel territory the excuse now is that the rebels should relinquish their land so that the Ukrainian military can safeguard the passage for investigators, do you think they will give the land back to the rebels once they're done?! No thought not. This is yet another reason in this evil game of brinkmanship that the Ukrainians stood to gain from shooting down this jet.