Paul fears Erik for a couple of distinct reasons. One of the most dominant is that Erik is so openly accepted by the family. Erik is seen as a "golden child" because of his charisma and his athletic talent. Erik's parents have become blinded by his athletic talent and his potential for the future. It is odd how blinded they are by Erik, enabling them to overlook his malevolence. Paul fears Erik because of this blindness, something upon which Paul himself comments: "But I can see. I can see everything. I can see things that Mom and Dad can't. Or won't." The odd way in which their parents view both children is one reason why Paul fears Erik.
Erik is oddly cruel. This is another reason that Paul fears him. Erik shows no reticence towards embracing violence and cruelty. The way in which he blinds Paul, the manner in which Erik uses violence without any guilt, and the destruction that Erik causes are all expressions of his cruelty. This level of cruelty and a shocking lack of remorse helps to make Erik odd. This oddity makes Paul fear him because there are no limits to what Erik can do. Erik has no problem committing transgressions and then openly lying about them in a believable manner. This is an odd behavior that Erik recognizes early on makes it difficult to challenge his brother.
I would also suggest that an odd aspect of Erik's behavior is his social acceptance. Paul is on the outside, and one who faces difficulty in social acceptance. By contrast, it is downright odd how easily Erik is socially accepted without a second though. Both Erik and Paul are new to the Florida from Houston. Yet, Paul is the outsider and Erik is so easily taken in as a social insider. This quick and immediate transition is odd, and reflective of an overwhelming condition of power that Erik possesses. It also represents much of the basic challenges that Paul experiences in the exposition of the novel.