Here's where it was shot down: https://www.google.com/maps/place/Torez,+Donetsk+Oblast,+Ukr...
Here's the territory controlled by the rebels: http://www.nytimes.com/interactive/2014/02/27/world/europe/u...
We know Russia has been supplying the rebels with heavy weapons. It's unlikely that Ukrainian pro-Kiev "neo nazis" marched a heavy SAM launcher through rebel controlled territory. Also, in any case, it doesn't follow that because someone is a "neo nazi" they will shoot down aircraft.