How does one interrogate a suspect that has so much power? If the local constables suspect an archmage of murder, what exactly can they do? A powerful wizard can pretty much use magic to hide his crimes, or resist any attempt of honest questioning.
I suppose the authorities can find a stronger wizard, but what happens if it’s the strongest wizard in the realm that is accused of wrong doing?
Do evil wizards automatically get away with murder in a fantasy setting as long as they’re the most powerful wizard around? Guess that’s one explanation for evil wizards trying to acquire more power…

not to be pessimistic, but even in real life rich or famous people get lighter sentences or no sentence for crimes. Big corporations can get away with fraude, tax evasion and massive polution, resulting in a lot of suffering all around, by greasing the right palms with, among things, “campaign donations” and such.
why would it be different in a fantasy setting? Who is gonna arrest the US president if he orders the CIA to assassinate someone? Who will arrest the evil wizard for what he orders his minions to do? supplant your powerful head of state of your choice and their executive arm, it still goes. China? Syria?
This is curbed by adventurers in fantasy settings. That’s what all those scenarios are about half the time…
Accuser: “Hey, you, adventurer, that wizard/noble/whoever over there is evil. I can kind of prove it, but not really. If I pay you gold, will you go deal with it for me?”
Adventurer: “How much gold? If the price is right, I’ll pretty much do whatever you want if it sounds legal and I can pretend like I”m good still while I do it.”
Now, if we only had adventurers who could get away with what we do in our fantasy games in real life.
I suppose there are several things that can happen depending on the setting.
In some systems magic leaves traces, in which would count as evidence.
Depending on the distribution of magic, there could be a group of less strong wizards who would be able to overcome his power with their combined magic.
About the vigilantes, it would make an interesting quest where a reclusive wizard is framed for a crime by another wizard, and hires the players to stop other groups of adventurers from breaking into his tower and killing him.
In unrelated news, this Thursday I played DnD for the first time. (ADnD First edition, for extra dork points.) I made a gnome illusionist/thief, and found out that attacks miss quite a lot (at least at first level). I also found out soon after we started playing that some of the other players rolled for stats eight or so times before sticking with them , (as opposed to my use of my lowish first set of rolls).
That evil wizard thing solely depends on the DM/story-teller I guess. Because if you so please, magic works this way, if you so pelase, it works that way, ’cause, you know “It’s magic!”.
The (N)PC can never be stronger than the DM – or so I heard.
But if we talk theoretically “what would it be like”; there could be all sorts of stuff, like enchanted interrogation-chambers where you can’t cast spells, or guards with spell-resistance-amuletts. What have you.
The reason medievalish settings are popular is that in the real 9th, 10th, 11th, 12th, and 13th centuries, justice, by and large, really was delivered by heroes, to the extent that it was delivered at all.
in my first ever dnd game, it was ADnD, one of my party had a riding/fighting dog. The dog was more deadly than all of us combined, which caused us to just hang back and watch the show
Those comments could make me think of a Bob Dylan song, “The Lonesome Death Of Hattie Carroll”… where another wealthy dude, William Zanzinger, who was arbitrarily responsible for her death, got a very light sentence for it…
“No matter how subtle the wizard, a knife between the shoulder blades will seriously cramp his style.”