Brother Leon is the Assistant Headmaster at Trinity, and Jerry's homeroom teacher. Like Archie, he seems almost too villainous to be believed. Since we never get inside his head, and don't get info about his past, we don't know if he's always been this way, or how he got so rotten. Seriously, this guy is just a few horcruxes short of being Lord Voldemort.
Leon just might be the biggest victimizer in the novel. He slashes kids with his pointer and practices daily psychological terrorism on them when they're in his class. Instead of, you know, teaching students, he exploits them for his personal gain.
And what about this whole chocolate sale thing? We learn that he's so concerned about the sale being a success because he didn't get school approval before spending twenty thousand bucks on leftover Mother's Day chocolates. If they don't sell, he owes the school that money. Well they do sell, and there's a twenty thousand dollar profit. We're not sure if he plans on taking the money himself, but we wouldn't be surprised if he tried to snag it.
The evil Brother Leon is also the main cause of Jerry's tragic beatings. Leon puts pressure on Archie to force Jerry to stop saying "No" to the chocolates. Archie wasn't really planning to harass Jerry beyond the initial assignment, but Brother Leon forces him to. Brother Leon's evil is confirmed at the end of the book when he watches the whole brutal raffle go down and doesn't try to stop it. On top of that, he makes sure that Archie gets off the hook at the end. So, yeah, you get the picture.