There were completely different opinions about that Rook's Rest scene. Someone said Aemond — above all else — thereby took revenge for the brothel mockery. Others said he took blood revenge for all that Aegon had done to him. It's kind of a solid reason, I mean, revenge, but I do not think it's the main one. There is an episode with Ewan Mitchell as a guest in The Official Game of Thrones Podcast: House of the Dragon. A really remarkable point of view was brought up there. Hosts asked mister Mitchell for his take on Aemond's actions, and I loved how he didn't give a straight-out answer actually and subtly turned the tables and asked hosts what they thought. They said that perhaps Aemond just taught Aegon a lesson. Literally: You're there, and this is what battle is like. Let's see how you do. I like that take. I like the way Aemond was all: You want to fight? I'll give you a fight.
Personally, I think — I've already written a short post about that — that Aemond burned his elder brother alive for inheritance. Aemond wants to be a king, but who he doesn't want to be is an usurper. He spotted an opportunity to put Aegon out of his way to the throne without being accused of wilful murder, and took a chance, that's all. Of course, he found some pleasure in this, in taking revenge. But first and foremost, to my mind, Aemond did this in order to become a king himself. That was a golden opportunity: Aegon's and Rhaenys's dragons flew at each other and engaged in a bloody battle, Aemond aimed at Rhaenys, but singed both, it was easy to write everything off as a tragic accident. Alternatively, if Aemond were called to account, he might to go to even greater extremes and say that he was trying to protect the king, that one second later and Rhaenys's dragon would bite Aegon to death, so Aemond, like a loving brother, a true patriot, sprang to save him, and it turned out how it turned out. Such an accident. It's very handy, ain't that the truth? Aemond has not fooled Criston Cole, though.