>>39962021Usually 3 Pokémon come up when we talk about Dark being the Evil Type. Umbreon, Darkrai, and Absol.
Umbreon is actually a baddie. Or at least can be rather mean/nasty. I know people think of it as cute, like all of Eevees other forms, but its dex entries do no depict it to be particularly kind, meek of tame. It's definitely not evil like other Dark Types, but it's probably the most likely Eevo to bite your face off.
The other two fit as Dark Types even though they are explicitly not evil. That's because they have powers over "bad stuff". Fire Types aren't LITERALLY made of fire, they have POWERS related to fire. This usually isn't the case with Dark, but for these two it is.
Darkrai gives nightmares unintentionally, but it still has "bad thing powers". Absol's whole thing is that it senses ill-fated events. When a disaster is about to come and kill everyonebody. So it senses oncoming misfortune, and therefor has "bad thing powers".
Plus Absol is a yin yang symbol. It makes sense to be the opposite of what it first appears to be.