I've seen a few pieces of scripture where the excuse for why the antag is evil is just "Well, he's really smart..." and then they go on to list various fake-sounding ideas about why that makes the antag evil. To be clear, being smart doesn't make people evil, it modifies their perception of evil, to the realization that there is no evil. There are merely conflicting views, and some insane people who have lost rational thought. There are many different views, usually relating to what people believe they need to protect. Synonymous with evil would be narcissism, which is pretty much what Stalin subscribed to. He thought that his views were more important than those of other people, and punished them brutally to remain in power. This is a minor facet of insanity. Hitler, on the other hand, was delusional, believing that he was helping more than he was hurting. in Mein Kampf, he even references himself as doing "god's work". Ultimately, definitions of right and wrong can be whittled down to personal beliefs in a functioning society. An evil anatgonist is likely either consumed with the belief of their importance, possibly without even realizing it, or they merely have an opposite goal to the protagonist. For example; in WWI (not WWII) neither Germany nor Britain was right or wrong. They were both defending their interests.