In general I think the issue is a lot of people equate Stalin with USSR. Things were substantially different both before and after him. And his reign was also from the 20s to the 50s in which there was the context of, amongst other major issues, WW2 where the Soviets lost tens of millions of people. As one can see in certain ongoing wars, exceptional loss of life seems to gradually push leaders towards having zero concern for life at all - let alone the liberties and values we hold to be desirable, even in authoritarian systems. When the "hard" decisions become quite easy, you're well on your way to dystopia.