Aaron Helzinger's amygdala (part of the brain) was removed in a failed attempt to cure him of his homicidal rage. This caused him to become even more excessively angry. Due to his childlike personality, he is easily manipulated by Batman villains. In the "Batman: The Last Arkham" storyline, Aaron Helzinger / Amygdala (manipulated by an insane Jeremiah Arkham) tried (and failed) to kill Batman. In "Knightfall", he was among the inmates of Arkham freed by Bane. Manipulated by the villain Ventriloquist, he tried (and failed) to kill Batman. He has been released and escaped multiple times. At one point he befriended Dick Grayson. In Infinite Crisis, he was a member of the Alexander Luthor Junior incarnation of the Secret Society of Super Villains. In Gotham Underground, he is seen at the Iceberg Lounge (a bar for villains made by the Penguin).
Amygdala has the of strength and durability of a bodybuilder. His emotional rages enhanced this further. These rages remove the restrains placed by people's self-awareness thus making him more dangerous.