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Wish Red Cross would provide some data, but nevertheless that's very interesting way of reporting, 800 cars left Mariupol, 2000 people made it through Russian-occupied territory to UA side. That's 2.5person/car. Either cars leaving besieged city half empty, cars are grabbing coffee taking their time and still driving through Russian side or ...
Factions of the Ukrainian side have been, since very nearly the start of this thing, using civilians as human shields as a strategy. By most of the interviews I've heard, civilians are reporting that the Russians have been fairly helpful in trying to allow them to leave the combat zones when they can. This makes logical sense, and most of the actual evidence that I can see backs that up. This is a good case-in-point where as the Russian gain control, freedom of movement seems to be restored.
And that's what it pretty much comes down to. Despite common sense, you have to make yourself believe that Putin, his generals, officers and soldiers all like killing innocent people just so much, to the point where they prefer taking cities with maximum amount of civilians still inside (despite troop morale, contrary to all independent indicators from Red Cross/UN, and lack of any military sense) because alternative would lead to accepting that a neo-nazi battalion is leveraging civilians to stall their own ultimate demise. Apparently some minds are just so set that they're just unable to accept that there are some bad actors in Ukraine.