All four of the prefects are corrupt to some extent, but Seth is by far the worst of the four. He is blatantly both corrupt and sadistic, and views his job not as a method of enforcing the rules, but an excuse to beat people. Indeed, nearly everything he says involves either wanting or intending to beat someone, and he expresses pleasure when someone finally slips up and gives him an excuse to do so. He frequently considers abusing his authority by picking on someone innocent. It's easily deducible that he was a Bully before becoming a prefect.
A possible explanation for his attitude is anger management problems; his dialogue implies that he received anger management counseling at some point, but he prefers to deal with his feelings by beating people up. He obviously has only contempt for the other students; he constantly refers to them, and Jimmy in particular, as "maggots" and "evildoers".
As Seth is in charge of supervising students on detention, tougher than the other prefects, and has the biggest role of the Prefects, he is generally thought to be the head prefect. The data files on Bully indicate that Karl Branting is actually the leader, and that Seth is a "helper", however, the data files are known to not be 100% accurate.