After Big D and A Le became sworn, Big D called a man to his bar, the man with sunglasses and pigtails. That man wants Big D to call A Le, kill A Le and get rich together.
As a result, this man was killed by Big D and A Le. After that, Big D and A Le held a party on the roof of Tsim Sha Tsui.
Obviously, the man they killed was the boss of Tsim Sha Tsui, their rival gang.