Evil is ugly

Note that "ugly" here is not to be taken too literally, as to some the "ugly" look can be "cool" instead, but generally speaking it involves deformities (e.g. horns growing, odd posture, etc.), discolorations (sickly skin, red eyes, etc.), and such appearing.

Easy to spot when all the good guys are pretty, handsome, beautiful, or at the very least normal looking while all the evil guys are somehow hideous, monstrous, deformed, or even something lesser like anorexic (skeletal) or morbidly obese, etc.