Fear dominates Paul's life. If Fear told him to do something, and he said "No, Fear, you're not the boss of me," Fear would say, "Oh, yes I am!" Ever since kindergarten, Paul has been afraid of his brother. Why? He's not quite sure. Fear of the unknown (like ghosts) is much worse than fear of the known (like Brussels sprouts), and poor Paul has been trying to understand his fear and remember his past for years. But Tangerine is more complicated than that: Paul doesn't mind causing a little fear himself, as a member of the War Eagles. So is there a chance that fear will turn him into another Erik?
Paul can only relate to his family through fear.
Paul's parents didn't punish Erik was because of their own fear. They thought ignoring Erik's behavior would make it go away.