Why does Johnny become The Red Chief?
Johnny is an unruly kid with too much energy and a wild imagination. He becomes Red Chief because it is all a game to him. When Bill and Sam kidnap him, he puts up a fight. But when they get him out of town, Johnny (Red Chief) immediately starts playing "Indian" with Bill. Sam notices that Red Chief doesn't seem at all worried that he's been kidnapped. Narrating, he notes, "Yes, sir, that boy seemed to be having the time of his life. The fun of camping out in a cave had made him forget that he was a captive himself." Sam even asks him if he wants to go home and Johnny replies that he has no fun at home and that he's having the best time of his life there at the cave.
Johnny continues to play Red Chief and Bill becomes mentally and physically exhausted because he takes the most abuse from Johnny's games. Johnny is impossible to control. He plays these games because he can. It is possible that Johnny is acting out because he is afraid after being kidnapped. But when Bill and Sam try to get a ransom, Johnny's father presents them with a counter-offer in which they pay him to take Johnny back. So, it would seem that Johnny's behavior is so bad at home that even his own father won't pay the ransom. Given his father's reaction to the request for ransom, it is likely that Johnny plays "Red Chief" or something like it at home as well. He's just a misbehaved kid. One comedic aspect of the story is that a child is too much for two seasoned criminals to handle.