keeping it very simple.
MH17 heading approx 100deg
Missile fired from ahead of it
Now going to the missile site
if manned by rebels they could erroneously think the target was a threat to them coming from that direction.
If manned by Ukraine forces why would they image anything from that direction (basically Kiev) would/could be a threat to them and therefore fire at it.
Sorry for all the technical details and fog on here, it was fired by rebel forces (BTW russians aren't that dull to have made that mistake)