He was captured by scientists in his world who implanted a chip in his brain. It shocks him if he tries to attack anything human. Tyki removed it here and that was what started most of the trouble that led up to his soul being returned. [Jack did you not mention any of this with the soul thing? Actually, the thought of Jack reminds him of something important.]
The chip doesn't work when he attacks Jack since he's not quite human. I'm not sure if it would activate for you, either, so be careful.