Those are two different questions>antagonist
There is really no rule here, it depends entirely on the specific work. >villain
They have to be fundamentally opposite from the hero in some specific thematic way. Whatever makes the hero, well, the hero of the story, the thematic centerpiece of the story, the villain does not have. Whatever difficult choice the hero makes, the villain took the other path. The heroe's weakness is the villain's weakness.
Luke Skywalker is the hero because he avoided the temptation of the darkside, and the paranoia and rigidity of the Jedi Order. He faced certain defeat and failure with love, compassion, and peace. Anakin is the villain because he gave into paranoia, he cut himself off from others, he fell to the darkside out of temptation, betraying the people he loved, and the people who loved him, because that was what he felt he had to do.
Jack is the hero of the story because he is selfless, pure good. He would go to no end of suffering and pain, in order to help others. Aku would cause no end of suffering and pain in order to help himself.