Wait, where did we find out that Aerion was the one who murdered Haegon?

“In 219 AC, Haegon Blackfyre and Bittersteel launched the Third Blackfyre Rebellion. Of the deeds done then, both good and ill—of the leadership of Maekar, the actions of Aerion Brightflame, the courage of Maekar’s youngest son, and the second duel between Bloodraven and Bittersteel—we know well. The pretender Haegon I Blackfyre died in the aftermath of battle, slain treacherously after he had given up his sword.”

Take a look at the parallel structures used and then extrapolate from what we know of Aerion’s character.