I say the villain is the reason why there is a need for heroes.
A yin for yang, tit for tat.
Villains can be a force of nature, an animal or a sentient creature.
Not all villains are evil, though, as the concept of 'evil' is as vague and grand as the concept of beauty.
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Evil people might not always call themselves evil. They would be wrong or simply lying to do so, but they might still deny their evil nature. Even the most deranged mass murderer might be able to justify his actions to himself in the name of his beliefs, his deity, or some skewed vision of what is best for the world.
Ex:
A killer might slay any children he deems weak or unfit to reach adulthood (similar to the ancient Spartan culture).
Another might kill children he believes will grow up and become evil themselves. Perhaps such a killer once had a prophetic dream telling him that evil was growing among the children of the town.
On a grander, larger scale, say, an evil priest might believe that to better serve his dark god, he needs to destroy an entire village and sacrifice all the residents.
Evil? Certainly. But does this priest see it as evil?
No, he sees it as a demonstration of his unending devotion and an aspect of his faith. Maybe he does see it as evil, too, but doesn't care.
A dictator might order the elimination of an entire race or members of a particular faith (Like Hitler's Nazi doctrines) because he believes them to be evil, or impure or imperfect. He might seek to dominate the world and bring its people under his unyielding fist.
Such a despot could believe though, that he is a good person and that the world will be better off without his guidance. This hypocritical ideal though, makes him no less a villain.
The above traits, my friends, are the true elements of villainy.
Peerless conviction, a self-serving conscience and the ability to discern (or fabricate, as is the common case) what is best for the world on his perspective.
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I am the villain who twists the soul and befouls the hearts of the purest heroes, that they may walk the path of ruin with their heads hung in regret and shame.