> When most people say Pol Pot was evil, they're not talking about demonic possession or some silly nonsense like that; they mean he was a mass murderer, which is evil.
Well, no. Most people I've heard say something like that would mean that he was innately evil as a person. They wouldn't spell it out like that but the underlying assumption is that a "normal human being" couldn't do what he did and therefore he was a freak mutation in some way. Most people even struggle with the idea that there was a time in such a person's life where they weren't "evil" yet. Even when talking about Antisocial Personality Disorder ("psychopathy") they rarely know that this is often in part caused by severe early childhood trauma, e.g. sexual abuse or parental abuse and emotional neglect - most seem to believe these people are "just born evil" and any prior period where they didn't do anything sufficiently evil were just a mask.
This is easiest to see when talking about the Nazis. Instead of trying to gain a systemic understanding of how the Nazis came to power or how "ordinary citizens" could be made to commit massacres and genocides we single out the big names as uniquely evil and make up excuses for the rest. For the longest time I had been told that soldiers who participated in massacres were implicitly threatened with death or at least physical punishment but we know that this was not the case and the mere threat of social ostracization by the other members of their unit was enough. The majority of those involved in the massacres saw themselves as victims for having to carry out those commands and deal with the trauma because denying their own agency helped them cope with what they had done.
So no, they don't mean "a person who has done something evil" when they say a person is evil. They usually mean something more transcendental than that. When most people say Pol Pot or Hitler or Milosevic was evil they mean he wasn't human the same way "normal humans" are human. They may not think he was literally possessed by a demon or the physical manifestation of a demon but they will think there was some essence of evilness inside him that would inevitably manifest. He wasn't evil as a result of doing evil things, he did evil things because he was an evil person. This is called essentialism and it's extremely widespread and antithetical to a systems theory based understanding of social dynamics and behaviors.