I think part of it is the environment anarkiddies and other radlibs spawn from. They almost universally grow up in the decadent west, where liberalism and cultural distortion has ground down their people so much that it can only produce leaders that are at best uninspiring and often loathed. Just look at today, the “leader of the free world” had to have military police and barbed wire guard his inauguration because people have him so much. That’s the kind of environment radlibs are used to, and due to cultural chauvinism they assume it applies to everyone else
Thus, when a truly great leader like Lenin or Stalin comes along, it completely shatters this illusion. It’s incomprehensible to them that a leader can be as strong, selfless, far sighted, and driven as Stalin was. So of course, rather than admit they’re wrong, the radlib doubles down and assumes that any leader that’s publicly loved by their people must actually be evil. It’s the only way they can reconcile reality with their belief that everyone is as mediocre and miserable as they are. Really it’s kinda sad in a way.