My understanding after watching all 12 episodes:

Baek family murdered Hong family successor (and almost In-a and Hee-joo) for practically no reason (seeing real Baek Sa-eon's face once), then Chairman Baek murdered his own grandson and put his own illegitimate child in his place. Once Chairman Hong learned about this, he threatened Baek family to obey at all times in order to keep these a secret.