They were never evil to begin with. Their sect originates from Persia, and have vastly different practises. People fear & detest what they don't understand. Even Ming Sect's pledge speaks of holy fire, light against darkness, do good deeds, rid the world of evil. Yes, Green-winged Bat King sucks peoples' blood. That's because of the martial arts he learned went awry. If he don't suck blood, he'll die. It was later in the novel that Wuji managed to cure him, so he no longer needs to suck blood.
