I think the writers were going for a more Hollywood!amnesia thing, but I'm going to hold on to the actual definition of a dissociative fugue and believe that he never really was that bad. For one, the personality that manifested is at complete odds with the composed and controlled persona that he had during his psychic days as shown in the flashbacks.

So Jane didn't revert to his actual old self, but rather to what he could have been, had he never had his family, or possibly to the kind of person he thinks he was back in those days (considering that he doesn't seem to have that high regard of his old self). Maybe even the kind of a person who he wants to be, a man who feels no guilt, who cares about nothing and no-one, who can have relationships without fearing that the other person will end up dead - a man who cannot be hurt again.