I agree with your point of view. I also believe that Kar’niss was turned sometime ago. The reason I believe this is because of how well he handles the body he now occupies. I imagine when a drow is first turned into a drider they struggle to navigate their strange, new eight-legged vessel and accidents could occur as a result. He seems very comfortable in his own skin, so to speak, and has no trouble climbing steep walls or wandering rough terrain. Kar’niss also has an aura about him that suggests age, even if we’re not talking elder years here. Something about his face speaks volumes, between the scars and the sour expression, this guy has been through it. Kar’niss doesn’t come off as fresh and new, but withered and worn. If he turned to the Absolute willingly it is likely because they convinced him they were his only hope, his only source of salvation. After many years of believing otherwise he opted to cling to the first shred of hope that appeared viable to him.

His psyche is a two-fold issue. Mix one part driderism with one part Absolutist mind breaks and you end up with a bleak mess of a beast. It’s also safe to assume that because he was a male drow he was subjected to untold abuse while in the Underdark, before and after the transformation. While we may never get the full lore behind why he was turned, I think there is a chance he cracked before he was transformed. It would depend on what level of pain and torture he was subjected to in the early years of his life.

Yes, I do think its bizarre that he holds that lantern so close to his face but he suffers from sunlight sensitivity. Even without the debuff, having something that bright close to your eyes would have to be blinding. I do like your take on blinded by fanaticism, it suits in this context! Poor Kar'niss will suffer discomfort if it means he feels protected and safe.

While we’re on the topic I need to bring up how both True Soul Nere and Kar’niss have sunlight sensitivity but the player character drow do not. I get that Larian probably removed the debuff from PC drow because a vast majority of combat takes place outdoors during the day and having a permanent debuff would make the race less appealing to players. But if Astarion’s sunlight issues can be removed by the tadpole, couldn’t the same be said for Nere and Kar’niss alongside PC drow? It’d make sense at least, but it was perhaps just an oversight.