As a lover of Dark type pokemon, I must ask: why is the Dark type called the “Evil type” in some cultures?

Cultural differences and beliefs, I’m afraid. Some cultures, either in their past or currently, fear dark-types for a number of reasons.

In some cases, people believed that these pokémon lurked in dark forests to attack humans … which, unfortunately, was somewhat true in olden times. Human settlements and roads were smaller and shorter in those days and did not encroach into wilderness as far as they do. So when travelers crossed through heavily wooded areas—places where pokémon such as houndour, poochyena, nuzleaf, and sneasel made their homes—these pokémon would attack them, thinking that they were prey. However, as time went on and as humans became more and more of a presence in the world, these pokémon gained exposure to humans (or learned about them from other members of their species) and came to realize that they weren’t to be attacked. Thus, wild houndour, poochyena, sneasel, nuzleaf, and similar pokémon are generally less likely to attack people than pokémon of any other type. Unless provoked, anyway.

(Cave pokémon, such as sableye, fall into this category as well, as humans have only recently figured out how to traverse cave systems safely.)

In other cases, people believed that these pokémon were omens or embodiments of death, usually because they would appear shortly before or after disaster. This is only true in a sense when it comes to absol. They can indeed predict disaster, and thus, their appearance should be an omen of impending doom. (And, in fact, they count on this, as they also realize humans don’t generally understand pokémon speak.) However, they don’t cause disaster, as many cultures seem to think. Meanwhile, pokémon like murkrow, vullaby, and zorua are all scavengers, and as such, they’re mistakenly thought to arrive before death, but in actuality, they arrive shortly after to feed on the fresh meat.

Finally, there are cases when the pokémon simply looks so fearsome and violent that it inspires abject fear in humans. Hydreigon is of particular note to this category.

As for why these pokémon specifically are considered to be evil when there are far more pokémon in other types who are associated with death or have attacked humans or are simply fearsome-looking, it’s just that the type contains the most pokémon who are associated with shadows. The elemental essence itself is based on darkness, and the techniques specific to the type often involve sneak attacks or vicious, bestial assaults. So humans have come to fear the dark-type in particular because of its associations with darkness.

If you ask me, though, anonymous, I quite agree with you: there is really nothing to fear from dark-type pokémon. Many of them are quite docile once properly tamed.