Extremists (aka Dark Heroes in foreign subcultures) are very similar to the anti-heroes, but what sets them apart, they are much more extreme in revenge. Moreover, many of them are almost on the verge of being protagonist villains, some make very dark and serious Moral Event Horizon acts as the sadistic torture relentlessly to making Mr. Krabs in his rival Plankton, who not intended to be funny or justified.

Perhaps these characters do a good deed done to stop by its rival, but his revenge against his rival / villain really take it to extremes. Not only the opponent or rival entry murder him, but also the physical or psychological torture, or rather they are a disgrace much more painful and cruel in their same opponents they played. It is the direct counterpart to their evil counterparts.

NOTE: Extremists are not Pure Good as they can commit unpredictable and terrible acts in the name of good, even if they are primarily focused on a revolution or a social change that is noble and meant to benefit the society.