One thing I haven't seen anyone sperg on is that he said, "The drawing shows a dog swimming in a pool."
NO! LIES! Nobody gives two shits if you go to the fucking Ducks Unlimited website and steal a picture of a swimming dog. He's lying in his own rationalization for what he did. Like if he'd straight said, "I had an idea for a cute picture and baby proportions are hard to draw so I used someone's kid for a reference," hey, that would be admitting that he knows why what he did was creepy and gross.
Another thing that nobody has mentioned, the dog's tail looks like a dripping penis. [ETA: A DRIPPING PENIS POINTED RIGHT AT THE DIAPER.] Dog tails do not look like this at all.

Finally, I don't trust anything troons say about their childhoods, their gender identity as kids, their likes or dislikes as kids, any abuse they may have suffered as kids, "playing doctor" with other kids, sexual experiences as teens, their parents' reaction to any gender non-conforming behavior...
Like one of my problems with the standard "serial killer" narrative is how they always say that their mother was a terrible person who physically and verbally abused them.
In some cases, such as Charles Manson, this was true and there is independent evidence documenting it. Kemper, on the other hand, said his mother used to lock him in the basement so he wouldn't molest his sister and this terrible treatment of him made him into a hater of women. Maybe--just maybe--his mother locked him in the basement because he was already a monster and she didn't know what else to do.
I would guess that most of these men were creepy little neckbeards when they were in first grade.