The Witch (Left 4 Dead)

The Witch is a special-type of Infected found in the video-game Left 4 Dead and normally is passive towards non-infected humans, however she is easily startled by light or any kind of aggressive behaviour - resulting in a sudden burst of violence as she goes on a warpath.

The Witch will only target the one responsible for "startling" her and is known to do so with incredible speed and accuracy - once she has locked onto a target she will not stop until either the object of her wrath is dead or she is killed: in some rare occassions she will target other survivors, normally if they continually shoot at her and thus distract her.

After killing a victim The Witch will run back into her hiding place and cry, suggesting she is remorseful for her actions but unable to stop the instinctual need to attack those who provoke her.