I think it's just the term "Evil Villain". No one ever classifies themselves as Evil or a Villain.
Maybe Maniacal Despot, or Tyrant, or something. But Evil Villain does have too many silly connotations attached to it, even if it does mean the same thing.
You caught it, it is specifically the "Evil Villain" part.
The issue, as with many things, is that the concept of a person who does evil things for the sake of doing evil things... and isn't played as a complete psychopath incapable of living in the real world, is in it of itself a silly concept.
So the only way to do it in a game where you are playing someone who is clearly evil just because is to just gloss over it or to make it incredibly ambiguous.
Another issue is that real heart wrenching villainy is rarely fun for a lot of people. Thus any game where you can play a villain also has to put a stop on how evil your character really is and isn't.
I mean, I know at some point there's just nothing left to do, or at least nothing that won't break everything else and require massive further investment, but most of them really do sound like meaningless fluff, especially if they're going to be releasing modding tools anyway.
Some I am fine with. The Co-op is something I starve for in most games. Especially story co-op.
Yet everything is yeah... useless useless fluff.
Reminds me of a D&D character I've been wanting to play
Ohh goodness if you want to see the quickest way to poor story telling you just watch an "Evil Campaign".
See dungeons and dragons really demonstrates how silly "Evil" can be since any evil campaign will quickly devolve into what I call "Psychopathy" or rather "Evil for evil sake". The game will FULLY support this idea that evil is overt mustach twirling villainy because it supports that acting "against" your allignment (in this case, not being evil) is a terrible thing.
AS IN! A evil person helping an old lady cross the street... Would litterally cry about it later.
This is supported by the fact that the game outright applies the very same limitations on good people doing evil things. A good person who is in a bad mood and kicks a lady trying to cross the street will cry about it later.
Play a game with "Evil" choices? The vast majority of them will be incredibly silly. Fable? Not doing anything but the most angelic of actions is considered absolutely evil and often to be "totally evil" you just have to kill people for no dang reason. You see there is no reason in evil.
Heck Play Dragon Age 1... Play with Morigan on your team and watch every single complaint, and she is the voice of chaotic evil in the game, at every single deed you do. Add Sten too. Litterally neither of them, because they are so dang evil, understand the concept of being likable, recompense, or even the concequences for their actions. Evil is silly! Evil you see has NO concept of doing good for any reason whatsoever because Evil is mentally disabled.
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Thus Evil is very silly... A Evil Villain is just silly
And if you arn't an Evil Villain you arn't evil you are just "Neutral".