I think what broke Aemond wasn't the bullying from Aegon and Jace, he would outgrow that eventually, as he likely did (we can see it when he's talking to Aegon during the dinner at the end of season 1).
What broke him, and took away all the empathy he could possible have, was the fact that despite doing something remarkable at such young age, no one cared. He's just claim the largest living dragon and the subject is Rhaenyra's sons. When Otto says he did good, he says to Alicent only and I doubt anyone ever said anything to Aemond himself.
I'm not defending his actions, just analysing the situation. When you get treated as a weapon long enough, you end up becoming one.












