Shen Zichen rarely gets mad. He prided that he had strong self-control and a mere insult wouldn't be enough to snap his patience. He developed the same temperament as his father growing up.
Since he was young, he cultivated this self-control and patience by dealing with his handful of younger siblings. This was why their parents trusted him to look after his siblings whenever they had to leave for business trips.
However, right now, he was livid. This despicable man continued to poke and test his patience over and over again and he just hit his bottom line. Li Xiuying was his bottom line and that was enough for his self-control to snap.
Shen Zichen would accept any insult or disrespect on his way but he would never let someone talk bad about Li Xiuying even if it was her own husband.