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This guy isn't Dark due to his reputation. It's because the Type is associated with misfortune and doom. Like a negative energy or black magic. That's common in Asian countries, the idea that wicked thoughts or unluckiness stems from a literal, actual negative force like a dark energy.
Dark Type's main defining trait isn't literally being evil, it's having "evil powers" like supernaturally-enhanced sneakiness, cunning or aura of intimidation. Being deceitful, hitting the low blows. Or some measure of control/affinity for a dark malignant force, like nightmares or catastrophes.
That doesn't make Dark Types innately evil, it means they are prone to evil behaviour because those behaviors tend to work complementary to their inborn affinity for all things grim and underhanded. You can still use stealthy/mean/dishonest/intimidating tactics explicitly for a good cause, like a spy or Batman or whatever.
Absol merely senses negative energy, looming catastrophe, like a Vaporeon senses approaching rain. Also Absol has a yin-yang for a face so the juxtaposition between its edgy appearance/powers and its selfless heroism is deliberate. It's supposed to be a statement on the intertwined nature of good and evil.
Its resemblance to the Grim Reaper is also deliberate, for either black or white are the color most associated with death depending on the country, and Death is likewise either a shadowy villain here to hurt you or here to save you and just doing his job depending on your point or view. IT'S GOOD SYMBOLISM PEOPLE
