Own the rights?
Not quite. Max Allan Collins was partially inspired by the Baby Cart Assassin film/manga series (a.k.a. Lone Wolf and Cub), but he based his story more-or-less directly on real events involving Irish gangster John Looney, federal lawman Eliot Ness and Chicago mobsters Al Capone and Frank Nitti. There was a real-life lieutenant of Looney who was betrayed by his superior.
No, he didn't take the money and that was the whole reason he killed Mr. Kelly. His message to the Rooneys was that he couldn't be bought off and would stop at nothing to avenge his family.
r43871