It is often said that Dark Type translates to Evil Type in Japanese, but looking at it thematically it's more like the Bad Guy or even just Bad/Wicked Things Type. I usually think of it not as the Evil or Dark Type, but as the "Dark-Hearted Type".
Fire Type Pokémon are not LITERALLY made of living flame, Bug Types are not always LITERALLY bugs, being more of a catch-all for small creepy crawly things, and Dark Types are not always literally evil or crooks.
The Pokémon mentioned in these discussions are usually Umbreon, Darkrai and Absol.
>Umbreon looks kinda cute, but if you read its flavor you'll find it does actually display some menacing, somewhat mean traits, so it fits as Dark. It also specifically is most active during the NEW moon, meaning the darkest nights. When it is hard to see, and the cover of darkness hides criminality.
>Darkrai is sometimes portrayed as evil, or not. But even when it's not, it still has power and control over a mythical wicked force, that being nightmares. It HAS "evil powers" even if it means no harm with them.
So Absol similarly has "wicked abilities" because it has a natural mythical sense that tells it when some terrible shit is about to go down. The word disaster comes from dis- + aster, meaning star. So it means something happened that was "ill-starred" or ill fated. It chooses not to use its powers for further evil, but it still is born with a supernatural affinity that tunes it in to cataclysmic. When something "goes wrong".
It likewise has Grim Reaper aspects to it, because disasters are times when people die. But Death is not always personified as a villain. Death can be comforting, even heroic in what he does. Absol being falsely SEEN as a villain isn't WHY it's a Dark Type, bu it befitting of is character.
Absol also has a Yin-Yang incorporated into is design motif. It actually makes sense that it looks like it would behave in one way, but actually behaves the opposite. Good and Evil are co-dependent.
