Damien was pissed at the thought of his father not doing everything he could for Amelia. If he were in that position, he would most definitely do something to protect the woman he loved. There was just no way for him to abandon Katherine over someone else.
If his father loved Amelia, why didn't he leave the household? How important was prestige for his father to be kept on a leash by the Park Family?
All this time, Damien thought his father was tough. That his father—despite the dark traits that he possessed—was a smart and formidable man capable of managing so many people and even ruling the underworld.
It turned out that he was wrong about his father. His father, Lawrence Park, was nothing but a coward.
A coward who couldn't protect his real family. An idiot who let go of someone he loved in exchange for wealth and power. Damien was enraged at the thought of his father choosing anything else over Amelia.