The appeal of playing evil is basically the appeal of how Hollywood sells evil:
Glamour. Power trip. You get to dress in black. Wreak shit. Enemies fear you. You fuck lascivious bitches who wear latex, and they want it. You conquer shit. Have a throne. Have badass capital starships and armies.
The reality of evil (especially as it manifests in our time, with no virtue mixed in at all, no martial virtue, which for example is in Darth Vader) is of course absolute shit, and nothing like this Hollywood image. Evil is being a smug police bully who batters European protestors, arrests old grannies for Tweets, but is a coward to foreign threats. Evil is a corpulent being like Jabba the Hutt, entitled to the point of demanding everything is delivered to them. Evil is an ugly politician that looks like a joke, and is also a pedo. Nobody "fears" these people, they aren't badass; they are a fucking disease.
Bascially all the glamourous bad boys, vampires, etc, are never how evil ever manifests. Hollywood probably reached peak power trip evil in the vampire craze. Be immortal, feared, etc, but also look like a Calvin Klein model for all time. What could possibly be an immature power fantasy there? It had a big influence, I think, on the current crop of rebels who imagine themselves as a transgressive vampire, and actually look like an overweight, disease riddled, flunky.
In my story I do not deal in Absolute Evil. I do not think there is such a thing, since that is Zero. I do not think that at any rate any ‘rational being’ is wholly evil. Satan fell. In my myth Morgoth fell before Creation of the physical world. In my story Sauron represents as near an approach to the wholly evil will as is possible. He had gone the way of all tyrants: beginning well, at least on the level that while desiring to order all things according to his own wisdom he still at first considered the (economic) well-being of other inhabitants of the Earth. But he went further than human tyrants in pride and the lust for domination, being in origin an immortal (angelic) spirit.* In The Lord of the Rings the conflict is not basically about ‘freedom’, though that is naturally involved. It is about God, and His sole right to divine honour. The Eldar and the Númenóreans believed in The One, the true God, and held worship of any other person an abomination. Sauron desired to be a God-King, and was held to be this by his servants;† if he had been victorious he would have demanded divine honour from all rational creatures and absolute temporal power over the whole world. * Of the same kind as Gandalf and Saruman, but of a far higher order. † By a triple treachery: 1. Because of his admiration of Strength he had become a follower of Morgoth and fell with him down into the depths of evil, becoming his chief agent in Middle Earth. 2. When Morgoth was defeated by the Valar finally he forsook his allegiance; but out of fear only; he did not present himself to the Valar or sue for pardon, and remained in Middle Earth. 3. When he found how greatly his knowledge was admired by all other rational creatures and how easy it was to influence them, his pride became boundless. - J.R.R. Tolkien
In KOTOR is easy to understand. Sith Lords have the Sauron thing going on. Angelic levels of malice refined by virtues like discipline. In D&D, where it amounts to taking money off orphans, I can't see the point.