An inquisitor is a sysop (or very influential user without formal sysop powers but respected by the sysop power structure, i.e. a personal friend of the GodKing) who is on a more or less continuous witch-hunt.
Such people are rare, but extremely destructive. They are the sworn enemies of most trolls, and the existence of even one of these people with any influence on the sysop power structure means there will be quite frequent ad hominem revert and ad hominem delete even by people who do not fully understand or agree with the reasons for doing this but who are afraid or unquestioning of the inquisitor or not being seen to be doing his or her will. Repeating his or her assertions and accusations can easily create an echo chamber.
Tolerating even one such person with any influence or credibility with the power structure creates a cabal almost by definition, and makes any large public wiki not troll-friendly. It leads almost certainly to libel and other legal problems, since the inquisitor is effectively setting editorial policy by intimidation, and becoming the de facto editor, and making the wiki legal owner the formal publisher of what s/he does NOT find objectionable... it's the level of influence and control over the content, and the ability to exclude contributors of something other than simple vandalism, that defines the role of editor.
It's the confusion of the ontological distinctions made by editors and the operational distinctions made by sysops, and the inability to tell one level of decision from another that tends to make the most mindless type of inquisitor, one who will simply decide "that is a troll" (someone they do not like, in this context) and perform an immediate ad hominem delete without even questioning their own ability to do psychiatry or resolve a political dispute that no one else on Earth can resolve. It's fair to say that this kind of confusion and habit creates far more problems than it solves.
So, an inquisitor is simply an incompetent editor who has no way to make an informed editorial decision, or no concern with the wiki's editorial mission or the end user's purpose or use for the information, and so relies only on contributor reputation to make decisions. This is fine to make friends of those who are in conflicts between users, but, it isn't what most wikis are intended to do, and certainly not what essential projects are supposed to do.