A villain who has an overall goal which the heroes can appreciate in principle, but whose methods of pursuing said goal (such as mass murder) are problematic; despite any sympathy they may have with their cause, the heroes have no choice but to stop them. Taken to extremes, they may fully believe that Utopia Justifies the Means. Such an idealistic extremist is likely to be either a Totalitarian Utilitarian or a Principles Zealot, depending on whether they’re aiming For Happiness or For Great Justice. The most well-written examples of this trope are the kind that the reader/viewer stops just short of agreeing with.
Other times, the villain may be out for simple revenge against a person, corporation or other entity that has undeniably wronged them. Again, the heroes may sympathize with their plight, but are obliged to stop them because they care not who gets in the way of their planned revenge. However, the heroes will often investigate the villain's grievance themselves and will complement stopping the villain with bringing the offending party to justice as well.
Maybe they started out with actions as good as their intentions, but were forced to take more extreme measures. Either way, it's a common end result of Jumping Off the Slippery Slope. Their favorite phrase is I Did What I Had to Do.
There are four basic flavors of Well Intentioned Extremist:
The problem is the means:
The Extremist has a good goal that many would support, given that it is fundamentally good and noble, but only the Extremist thinks the means they have chosen are acceptable. Perhaps this particular road to utopia is Powered by a Forsaken Child or simply includes a nauseating level of so-called 'necessary' sacrifices.
The Extremist has a good goal that many would support, given that it is fundamentally good and noble, but the means the TU has chosen have hidden/'hidden' costs. Though it'll 'solve' the original problem, it'll bring about something even worse. For example, they may try to prevent crime... by sacrificing everyone's souls to an eldritch abomination so they will no longer have personalities or feel the desire to steal. They may not have even realized that this is a problem.
The problem is the goal:
The Extremist has a goal that few would support, given just how extreme and illogical it is. Any principle can become Nightmare Fuel if one divorces it from common sense and basic human decency. For example:
We should maximize the average happiness. This can most easily be done by constantly, and secretly, exterminating the unhappiest people.
We should maximize the total happiness. Since even starving people are capable of happiness, we should raise the total population as high as possible, regardless of each individual's quality of life.
Negative utilitarianism. Instead of maximizing happiness, we should only focus on minimizing suffering. However, since all living things suffer to some extent we can only eliminate suffering by putting the unhappy out of their misery - or by exterminating everyone.
We should just make everyone happy; people who are unhappy should be cured of this condition using the latest technology or magitek.
The problem is the consequences:
The Extremist sees a real problem and offers a real solution that may or may not be ethically acceptable, but the bigger issue is that they are not taking into account the negative side-effects of their actions. For example:
The Extremist wants to assassinate the evil dictator of an oppressed country, but can't or won't take the time and effort to ensure that the country is not taken over by another who is just as bad or worse, or perhaps the country falls into chaos without the tyrant or a stronger, benevolent replacement to hold it all together.
The problem is the "problem":
The Extremist has a goal that more or less everyone can agree is misinformed, insane, or both — they're willing to make (huge) sacrifices in the name of a completely useless goal. For example, they might try to fix the endemic and civilisation-destroying catastrophe of ant-theft... by organising the sacrificial murder of the entire population of New Delhi as an offering to a non-existent supernatural entity that they think will solve the problem with its magic.
On the extreme end, may result in the even darker Knight Templar who chooses to either remove free will, believing it the best way to save the world or destroy whoever they don’t consider worthy enough to live. Unlike this, The Well-Intentioned Extremist isn't a punisher of all flaws or a dictator who argues that they are good and therefore everyone else is bad; they actually realise that some of their victims don't deserve to die or suffer, but insist that it is for the greater good and actually isn't entirely wrong. They could also be a fallen Wide-Eyed Idealist that was driven over the edge, and sometimes a Worthy Opponent or even Reluctant Warrior. Vigilante Man is a case where the Well-Intentioned Extremist hasn't (yet) descended to the point of not caring who gets hurt. Often ends up in rivalries with the Knight in Sour Armor. Maybe they're Obliviously Evil and don't realise they're the bad guy, in which case they're prone to My God, What Have I Done? and becoming wracked with guilt and horror if they're ever enlightened. Some of those seeking to bring about a One World Order to end international strife may count as this.
This can be a difficult line to walk: push too hard in one direction and you make it seem as if the extreme methods are acceptable; push too hard in the other and you run the risk of denying the validity of their initial cause, of arguing for oppression (for instance) instead of against it. When the balancing is done they more often than not tend to be either Anti-Villains or even dark Anti-Heroes, especially if put more emphasis on the "well-intentioned" part than the "extremist" one. However, there are some who seem like WIEs but in reality are just straight-up villains, particularly if they are the other way around in terms of what they put more emphasis on and if they are obviously just using their philosophy as a shallow excuse to commit evil. Another version of a darker villain being this trope is that they have an actual philosophy which they believe will "improve" the world but the philosophy itself is too obviously evil to garner any sympathy, such as is often the case with the Misanthrope Supreme and genocidal maniacs (especially those who believe they're in a Guilt-Free Extermination War and act accordingly).
Often, these characters are presented as more dangerous and terrifying than some who are straight-up For the Evulz and Take Over the World types and with good reason: Even Evil Has Standards does not usually apply to them, as it might be for some others. After all, if you think you're doing the right thing, then it's easy to consider even the most horrific actions an acceptable compromise.
Compare with Necessarily Evil, where the villain in question has a Heel Realization and recognizes that he deserves punishment (of course, he may always choose to just Ignore That Epiphany). See also A Lighter Shade of Grey. A staple trope of the Master Computer gone mad. If the positive intention is overthrowing an evil government, the Well Intentioned Extremists will be an example of The Revolution Will Not Be Civilized. If his extremism actually succeeds in making the world a better place, it's The Extremist Was Right. The Small Steps Hero desperately wants to avert this trope. The Complete Monster will instead subvert this trope.
As this trope often goes hand-in-hand with the Face–Heel Turn or may involve the reveal of a villain's true plans (or intentions), expect spoilers.
