Complex sentence: His instincts tell him that something is wrong about Cho, and Kang starts stalking him around, soon getting convinced that Cho is the murderer, while no one else believes that he has a case.
Split into simpler sentences: His instincts tell him that something is wrong about Cho, and Kang starts stalking him. Kang soon convinces himself that Cho is the murderer, although no one else believes that he has a case.