One of the best videos on my YouTube channel is called Satanic Banality. It’s only tangentially about what is commonly called “the banality of evil”. It is really about the concept of “satanic grandeur” and how evil always looks small from the outside, it only looks impressive from the inside. If you haven’t watched it, I’ll embed the video in case you’re interested:
The applicability to writing is in making realistic villains, and especially in showing that they’re evil in realistic ways. In daytime cartoons for children, villains are shown to be evil by just choosing to be evil for the sake of Evil, then laughing about it. This works well in cartoons for children because children need things intelligible more than they need them realistic. Children only pick up on the broadest strokes, like that evil should be opposed. For children, at the level they understand them, cartoons are actually highly realistic.
In fiction for adults, who understand the narrative in much greater detail, we need it to be realistic on all of the levels that the adult reader understands it. This is the real reason for the addage “show, don’t tell”. It’s not that there’s anything wrong with telling the reader something, it’s just that if you do that in place of showing, you will have a contradictory character. If you tell the reader that the man is evil but then show him being humble, gentle, courteous, kind, thoughtful, brave, reverent, etc.—this is bad writing because he isn’t actually evil.
This brings me to satanic banality as a writing tool. Something important to remember about evil people is that they are always vastly more impressed with themselves than everyone else is. This does not mean that they are vain. They may or may not be vain—vanity consists in wanting others to recognize their greatness. But whether they are vain, they are quite impressed with themselves. They think that their vices are actually virtues. This comes from the nature of evil.
Evil is not a positive thing, but a privation of good. It is like a shadow cast by being; it looks like it has a shape, but it has no actual substance. A person who is evil is trying to act like the shadow is real—as if it can be touched and picked up and used. Thus, to them, what they do when they are evil looks magnificent. It must, or they would not try to do it. To those who see the shadow for what it is, they look banal. To somewhat mix a metaphor, imagine someone shadow boxing who thinks he’s beating the shadow that he’s hitting, and is therefore a great warrior. In his mind, he’s magnificent. To those watching, he cannot be impressive, because they can see that he’s not hitting anything.
This does not mean that the villain is bad at everything he does, of course. It means that the villain is going to think himself grandiose precisely where he is evil. He may not care whether others think he is great—that is, he may or may not be vain—but he will at least expect that they will think him great, or will expect that the smart ones will. This will be one of his weaknesses, since he will be wrong. (If he’s not wrong, here, it will be because he thinks that there are no smart ones, which will be a different sort of weakness.)
The other thing is that the villain will do things that just seem absurd. He will make statues to himself for things that he didn’t do, but thinks that he did. The Kim family in North Korea is a good example of this; you can find monuments they’ve built to themselves about how much of a champion of the people they are, what great movies they’ve made, etc. They are an extreme example, but it’s not hard to dial this back if one wants a less evil sort of villain. A warlord who wants to take over the world will think that he is bringing peace, and may well build statues to himself as a protector of the people. A cheater at sports who has not been caught will think of himself as as great role model for children. A thief will think of himself as enforcing justice, being a Robin Hood who robs from the rich and gives to the poor (in this case, himself). (They never consider that Robin Hood actually robbed from a rapacious government to give to those who were overly heavily taxed.)
These sorts of mistakes are often confused for rationalizations, that is, for excuses made to others. This is to mistake the nature of evil. The evildoer really believes these things, precisely because in his sin he has missed what he’s aiming at. When trying to write a realistic villain, this sort of mistake is not optional. Villains are villains precisely because they are wrong about some moral judgement. These mistakes will have consequences beyond merely doing evil, precisely because the villain actually believes these moral errors. Working these sorts of systemic errors into the villains actions will make him far more realistic, as well as adding a great deal of depth and insight into the story.