system
Given the following post, respond as if you're commenting on Reddit:
human
[Witcher series] Is the man of glass a powerful being that enjoys manipulating and torturing humans just for fun? Or is he the "primevil" of this universe that has an overarching purpose for doing all these things?
gpt
He's *incredibly incredibly likely* to be The Devil incarnate. A lot of people who talk about him suggest he's pure evil, and that he conducts Faustian Bargains (essentially deals with The Devil). 

Given his demonic appearance when beaten at the end of Hearts of Stone and the demonic chanting (apparently translated back, he is saying something along the lines of always finding a way back into the world) it matches up quite eerily to the suggestion he is The Devil. Like The Devil, his bargains always screw over the other person in some way, and end with Gaunter o'Dimm (The Devil) claiming the person's soul, unless someone can bargain with *and then best* The Devil in a game of the latter's choice. 

Master of Mirrors is also a symbolic term. A mirror shows you a reflection of yourselves, but also is attributed to a version of you that is morally inverted to yourself. As most people are somewhat good, a Master of Mirrors likely deals with the evil inherent in beings. He's also one of the only beings in the Witcherverse said to be unkillable, and never able to be beaten for good. Perhaps this is because Gaunter is such a powerful entity whose weakness hasn't been discovered yet? Or perhaps it's because he's The Devil, and while evil lurks in the hearts of men (or Elves, Dwarves, Dopplers, Monsters, whomever) then he can always return to the world. 

Although he does find some enjoyment in the suffering of mortals, his end goal is to gather as many souls as possible, likely either to add to his power *or* in preparation for some kind of Judgement Day event, when the forces of the God(s) clash against his own, and he needs all the souls of the damned he can muster for his side.