A common fallacy in stories, particularly fanfiction and often crossovers, is the 'worse evil' fallacy.
What I mean is when a villain's evil is measured by being categorized as being worse than another simply because their plans have a greater SCOPE than another villain is CAPABLE of.
This is often described as a character sensing 'an evil worse than X villain they'd already faced'.
Now, this sometimes works if the villain in question actually IS worse in both scope AND evil, but often, it's merely the first one.
The reason this is a fallacy is it only takes into account the harm the villain is currently CAPABLE of, not how evil they ACTUALLY are. Their motivations and how black their soul is.
In other worses, it measures their 'evil' in terms of the damage they can cause, not how vile they are. Ie, measuring in Hiroshimas and not (to quote Linkara) Megahitlers
A good example of the fault of this logic is Kamen Rider Ryuki:
The main villain is having 13 people fight to the death