Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Many characters in Rabbit, Run say, do, and think harsh things. But a tone of sympathy, and even love pervades. And man are these folks judgmental. Like when Rabbit calls Janice stupid, or when he calls himself a criminal. The tough talking narrator, though it seems to call for a complete overhaul of social norms, also seems to suggest that we are all just people, and people make mistakes. When we are able to identify with flawed or disliked characters, we can sometimes gain real introspection, as well as a deeper understanding of those around us.