Unlike Slim, Dr. Brennan encourages Eli to see that good and bad people are totally different. In her view, good and bad is a binary, not a spectrum (which is what Slim has been pointing to when he talks to Eli about good and evil). Dr. Brennan clearly wants to know the truth about who cut Eli’s finger off, and she doesn’t believe that August did it. Telling her the truth, she implies, is the only way that Eli can be a hero in this scenario. But although Eli does seem to agree with her that good and evil are a binary, his loyalty to Mum and Slim means that he doesn’t deviate from Mum’s story.