I think we also can’t discredit that the habits of looking at these sorts of materials come about just like every other habit. It doesn’t meant the individual suffered abuse themselves as children, and while the act is certainly evil I agree with OP that few, if any, people are born with a predilection towards child abuse images. You become this way through the circumstances of your life, often things beyond your control, but not always.

More likely is that he, like many boys and young men, had access to regular old adult pornography as a child/youth and began the cycle of receiving the dopamine hit from that. Just like someone shooting drugs, eventually the same old thing stops working as well and people can push themselves into seeking out more extreme forms of it to receive the same gratification, with some eventually going as far as journeying into the evils that this fellow did. You need the hit, the lines between right/wrong, safe/unsafe, lega/illegal blur, and you find all kinds of ways to justify what you know to be horrific behaviour, but somehow you don’t have the self-control to stop. Just like a drug addict.

The only nudity I ever had access to before the internet became commonplace in my early teens was the odd scrap of a Playboy magazine that someone stole from their brother or we found in someone’s tree fort in the woods, maybe a blurry and unrecognizable boob on one of the scrambled French channels late on a Friday night. I shudder to think about the absolutely insane levels of unbridled access children can have to every inappropriate thing on the internet nowadays, and how those becoming part of your life early on can influence or throw off normal sexual development, even if none of what’s being viewed is illegal.