Extremists are perhaps some of the most dangerous villains one can encounter; they are obviously criminal, malicious or delinquent in action, supporting beliefs totally at odds with what most find a "normal" or "fair" society yet they have not (at least publicly) crossed the legal line into outright terrorism.
Being the opposite of delusionals (who only think that their goals are in the right), they are villains who have well-intentioned goals for the greater good, but whose methods and ultimate ends are very extreme, to the point that the heroes have no choice but to stop them. Sometimes, extremists can go too far with their actions, to the point where they cross the Moral Event Horizon and become straight-up delusional villains who think that they are saving the world by removing free will.
Extremists' schemes will possibly be immoral, such as framing, kidnapping, murdering, covering up, manipulating, psychologically abusing people, emotionally blackmailing, and even using every single psychological manipulation tactic there is.
Although monitored heavily by authorities and often despised by the public, these Extremists are dangerous and unsettling in the fact they are working within the law (albeit the very fringes of it) and may have many supporters for their cause, although they sometimes work for themselves and do not work directly under the government.
Religious extremism, racist organizations that do not become outlawed (in their native country), and individuals/groups who hold extremely controversial, often hateful views yet are not officially considered terrorists can be added here.
For more information, read up on Hate Groups. Those listed as "Non-Outlawed" would be acceptable here, as would some extreme "fringe" groups, non-outlawed cults and so forth. Do NOT use this as an excuse to create propaganda designed to demonize a certain group or people; they must have established facts showing they are what most would consider a Destructive Cult or similar.