Anti-Villains is a trope popular found in any media is about a character who with a double sense of moral yet the have the same or equal degree of action like a hero.
Who are they and what made them so special from other regular villains?
To Clarify an anti-villains is not equal at all as anti-hero, An Anti-Villain is the opposite of an Anti-Hero; while the anti-hero often fights on the side of good, but with selfish motives; the anti-villain plays a villain's game, but for a noble cause… at least in their eyes. They may be more noble or heroic than an anti-hero, but the means to achieve their ends are often considered immoral, unjust, or evil. Sometimes, they may simply be a villain with gentlemanly qualities, a code of honor or some sense of justice. Anti-villains will occasionally side with their rivals (usually the protagonist) if a greater threat than themselves comes or it's in both of their best interests. Often times, if they do join the protagonists, they are distrusted. These villains are also often considered grey characters due to their constant moral ambiguity. It is possible and in fact very common to be both an anti-villain and anti-hero at the same time, depending on the character's goal and their methods of attaining it.
In essence, these are characters that although technically villains often serve in a noble capacity, which makes them sporadically heroic. They also can be in the center of being in the Grey Zone depending on their alignment. Alternatively, characters that are meant to be seen as villains in universe or out, but are not really all that evil, such as a Designated Villain or a Punch Clock Villain, may also be this.
In fact, Usually this kind of villains is not always as bad some of them are Villains by Proxy or Tragic Villains (Haku is a good example of this), others are well-intentioned characters who want the society evolve in a utopia or more or less and equal status quo to everyone (E.g The Punisher).
Examples of popular anti-villains include Magneto from the X-Men comic book series, Mello from Death Note, Captain Hector Barbossa from Disney's Pirates of the Caribbean, Stewie Griffin from Family Guy, V from V for Vendetta, Jin Kazama from the Tekken franchise, Satsuki Kiryuin from Kill la Kill, Agent John Bishop from the TMNT franchise, Itachi Uchiha from the Naruto series, and the Imposter Dan from Dan Vs..
NOTE: Do not add Complete Monsters in this page as they are eternally evil, and they no have regret in his actions against them all, as well as they are never presented in any positive light.