I'm not sure you could faithfully adapt it to a game, or if it needs it. I just couldn't see there being much enjoyment in it when so much of the book is basically one-sided slaughter. A film would perhaps work.

If you were to do a game, I think something more like a separate story set in the same world would work better. Some other gang influenced by the Judge in which morality plays a large part, and you might gradually feel yourself being corrupted by the Judge depending on your actions. I'm imagining a Mass Effect-esque western shooter with lots of quests.