isnt cargo aircraft "off limits" ? unarmed ?
In a war situation I dont think that has ever been the case. I once researched a wartime Beaufighter pilot, a DFC and an accredited "ace". Three of the kills in his tally consisted of a flight of unescorted JU 52s which he encountered over the Med. They could have been carrying vital war supplies or troops to North Africa - either way he had no choice but to shoot them down, although I don't suppose he took much pleasure in doing so.
Here however we have an undeclared war, and I doubt if the pro-Russian "troops" who shot this aircraft down give much thought to the Geneva convention.