Why is everyone hating on Lee Kang? They are only in high school. Should he have acted like that? No. But he's just a teenage boy. He thought Juheeon (or however you spell the MC's name) was basically cheating and got the girl pregnant. When the MC went to tell him about the baby, he overreacted, and didn't fully listen. So Lee Kang thought the MC was telling him het got the girl pregnant, not that Juheeon himself is pregnant by Lee Kang. Don't forget that everyone, including Kang and Juheeon, thought that Juheeon was a beta. He was acting very childish, but that's no reason to e hating him so much. He's still a good guy.
~ Unknown