Conduct Zero(2002)

When Joong-pil, the biggest bully in his high-school by far finds himself harboring feelings for a classmate, his entire identity is thrown off balance. Suddenly his smoking, his drinking, his constant swearing and his habit of pushing underclassmen into walls don't seem like good habits anymore. Without his nastiness, who is he? He might just like this girl enough to find out, if he can figure out how.