Qui bono?
I am rather surprised that nobody mentioned the simple reason why would rebels want to shoot down an airliner:
They don't have air force thus anything in the air is either neutral or an enemy. After MH17 the air space is closed and anything that moves is a free game for their air defence.
Another reason would be to escalate the conflict per se. Even if supported by Russia now they can see that Putin is apparently satisfied with holding Krym now in his hands and nobody is talking about annexation anymore and there is enough stuff for international media to talk about. Krym is still the main issue and it is forgotten now.
It seems that Putin is not willing to use the military force to engage directly but is aiming similar status quo like Transnistria. But Moldova and Ukraine is quite a difference and rebels may try to escalate conflict to bait Putin. In the beginning Putin made them believe he would back them up directly but in my opinion he never intended to acquire more than Krym and to destabilise Ukraine.