Just as Good characters aren't allowed to have Bad Powers, Evil characters are not allowed to have Good powers -- or, more specifically, healing powers. In short, characters on The Dark Side are generally incapable of using powers, skills, spells, or abilities that would serve to restore or protect rather than destroy. This trope has nothing to do with actually "curing the evil". For that see Heel Face Turn. Contrast with Good Powers, Bad People, for villains who not only get "good" powers, but use them for evil. See also Harmful Healing for one application of aversions to this trope.