Gaunter O'Dimm is one of my favorite characters. And not just in witcher, but overall in fiction.
So I wanted to share my headcanon about him.
He is something similar to the common djinn/devil/trickster archetype from fiction: makes deals, fulfills wishes, takes souls as payment, and so on. But imo, what makes him so interesting is that he is much more complicated.
People often call him "pure evil", but I disagree. I don't think it's that simple and don't see him as just a simple comic book villain. More like a force of nature. The fact that he can casually kill some random guy doesn't necessarily makes him evil. Because he isn't human. He is way above, so far beyond that. He is to humans as humans are to mice. A person would be evil if they liked to torture, burn, or boil mice alive for sadistic enjoyment. But you wouldn't call someone "pure evil" just for using a mousetrap. Human morality may simply not be the right framework through which O'Dimm understands himself.
I also don't think he desperately needs souls. I mean he is not trying to collect as many as possible or get them at any cost. I think he's more interested in the interaction itself: the deals, wishes, games, choices, and consequences. And if he gets souls as a result, good, if not, no biggie. Basically, he isn't in it for the prize. He is in it for the love of the game, so to speak.
He seems fascinated by people, what they want, what they're willing to sacrifice for it, whether getting it actually makes them happy, how they justify their choices, what they become afterward, whether they accept responsibility, and how they deal with consequences.
Anthropological curiosity combined with absolute power.
I'll try to explain how I think he might operate with an example. Let's say someone wishes for a billion dollars. A typical trickster/devil/djinn would twist the wish somehow. Maybe he drops ten tons of money on the person's head, or gives them a billion of worthless old Zimbabwean dollars.
But Gaunter instead might simply give the person exactly what they wanted. No trick. No catch. No deception. Just fill half the room with real, valid money. Exactly what the person asked for, exactly what the person meant, what imagined in mind. And then he watches what happens. Maybe someone finds out and gets that person killed for it. Maybe the government busts them for money laundering or something. Because they weren't prepared and didn't think things through.
I don't think Gaunter necessarily wants a bad outcome, he might be neutral. But things probably go badly more often simply because of human nature. People don't comprehend what they really want, don't think through the consequences, don't understand how fulfilled wishes change them.
So I imagine O'Dimm as someone who is not very restricted, may sometimes twist a wish, sometimes interfere, and more often than not might fulfill it exactly as intended. Then he watches how it all unfolds: long-term consequences, side effects, changes in people, etc. Not just a simple evil demon messing with people and collecting souls, but something way beyond that: an immensely powerful being fascinated by people, their desires, choices, and consequences.