The expression of the Dark (Evil) type varies a lot based on the Pokemon in question.
Weavile and the like are actively cruel and take distinct pleasure in harming their prey.
Yveltal is a legendary pokemon that represents the destruction wrought by natural disasters. It could be considered the equivalent of a living forest fire but serves as part of the life/death cycle of renewal with Xerneas, the Life Pokemon.
Darkrai is sometimes portrayed as a rather miserable Pokemon, tied by its very nature to nightmares.
Absol has a reputation for bringing about disaster but that is due to it having a precognitive sense of danger. Not necessarily the cause, just danger. As such it is often reviled by those who are superstitious.
Houndour/doom are an interesting case, being among the earliest dark types introduced during the second generation. Their fire is especially harmful due to the toxins in their bodies, inflicting pain that is said to be eternal. They are pack hunters who, according to their pokedex entry, evenly divide what they catch amongst each other and even their leader does not take a larger share.
Perhaps the best way to describe the dark type is 'Unfair'. They learn a significant number of moves that are meant to manipulate their opponent into behaving a certain way or else reduce their opponents ability to fight. Not necessarily dark type moves, either.
Sand Attack and Mud Slap are both used widely among the dark types and Houndour in specific can also naturally learn Smog (Poison type, chance to poison), Incinerate (Fire type attack, destroy opponent held item), and Foul Play (Dark, attack using the opponents power instead of your own). Whilst the dark type does have a significant number of direct attack options, many of their best plays involve manipulation instead.
