Danny is not the champion of the world in a pugilistic sense; he is not the heavyweight champion of the world, or a fighter like Muhammed Ali. He is more like a people's champion, a twentieth century Robin Hood who steals from the rich, not so much to distribute to the poor, but to humiliate the people at the top of the tree whom they feel flaunt their wealth and good fortune whilst others in the community are struggling.
The pheasants that Danny and his father steal are a case in point. Danny's father poaches pheasants, which means that he steals them from someone else's land. He cooks them, and relies on them sometimes to put food on the table. Mr Hazell uses the pheasants for recreation and his idea of fun and entertainment is inviting his friends over to shoot the pheasants clean out of the sky. This shows his complete lack of character and morality, and it also shows the gulf between Hazell and the rest of the village. Danny's father owns his own business, but he still struggles, and to see Hazell shooting birds for fun when they could be used to feed some families in the community really makes his blood boil.
Dahl uses this situation to ask questions of his readers; should we really be on Danny's side, considering he is stealing from Mr Hazell? Is Danny's father any more ethical than Mr Hazell, because he, too, is happy to kill the pheasants, and he also steals from the local landowners with his poaching activities. In the end, it is clear that all of the characters in the book have a flawed sense of right and wrong, but we find ourselves siding with Danny and his father, because Mr Hazell is a pompous show off, and inherently less likeble than a single dad who is trying to provide for his son.