You seem to imply that it’s unlikely that the missile was fired by Russians or separatists, leaving Ukraine as your unnamed suspect.
In my view the most likely scenario is an undertrained and overeager separatist crew that was provided with weapons by the Russians. If the rumors that Russia is destroying evidence turn out to be true, then Russia is de facto admitting 100% blame for this incident no matter who pulled the trigger.
I also believe that to be a likely explanation.