So Sonny shot AJ because he thought he was choking Ava and well because he kind of hates him. Sonny and Carly have good reason to hate AJ. He was responsible for the accident that left Jason brain damaged and without his memory. He was responsible for Sonny and Carly losing their first baby. He kidnapped all THREE of Sonny’s children and terrorized them (especially Michael). Sure AJ was never smart enough to create havoc on a colossal scale such as Ric. AJ is more of a stumbling doofus kind of bad guy.