Rationality can often allow us to overcome otherwise debilitating emotional responses. I think a non-rationalist in the same situation who let their psychopathic brother die ... they would probably feel a lot of guilt. A LOT of guilt. Finally, evolution isn't always fine-tuned, especially in social contexts. Guilt simply may not be a fine-tuned enough emotion to make you feel pain directly proportional to the odds of you being suspected for a crime; it's more likely that you feel guilt differently in different classes of situations rather than in different levels of the same scenario.