He's an asshole because of other things outside of his baseball efforts. Like the betting with Chihiro as a prize, thing. Raging out like a lunatic didn't help either. Then getting popular and arbitrarily deciding he doesn't need her anyway so the bet is off. You know who throws their feelings at someone else, demands their attention, and then disregards them entirely is? Yeah, an asshole. Being good at baseball through his own efforts and becoming popular as a result is completely irrelevant to how he behaves as a person.