SPOILER
Hints can be found in his fight with the circus master Barmunk in volume 3 that some sort of a demon resides in Akutabai's body. From their conversation, we gather that this demon threatens to take over everytime Akutabai is agitated but at the same time, it gives him almost close to invincible strength and speed. Because of a promise to a long haired woman (we never see her face, personally it reminds me of the first few glimpses we see of Rem in Trigun through Vash's eyes) Akutabai has sworn to lay to rest that part of him.
Although Barmunk taunts Akutabai saying that he should not and cannot suppress the demon, the demon being the source of power, Akutabai replies "No, if I will to call on him to fight you now, I merely give in to insanity. If I call on him to protect the innocent, that is true power".