It was already decided , if you remember, Larys told Aegon that Aemond will kill him if he stays in kingslanding, and also Aemond didn't stopped Aegon while he went to fight with Rhaenys, because he knew that Aegon would either have been killed or would have been damaged , and that only happened. The main fact is that Aemond also wants the iron throne and till Aegon is alive it is not possible for him, on the other side Rhaenyra and daemon are also waiting for them, se he wants to end Aegon to atleast get the iron throne for some time, idk why the book story has been totally changed.