One Nite in Mongkok (2004) on Netflix

Hong Kong nihilism. December 22, a street quarrel leads to the death of a gang leader's son. Next day, he seeks revenge on his brother, a rival boss. He calls on Liu, a fixer, to import a hit man from the mainland. Lai Fu, a tough and youthful hick, arrives with a day pass. The cops, led by the morose Milo, hear about the killer; they open a full-scale Christmas Eve operation to find the warring brothers and Lai Fu. Lai Fu rescues a hooker, Dan Dan, from a sadist and asks her to help him find his way around Mongkok. By nightfall, Liu has double crossed Lai Fu, the brothers are hiding, the cops are everywhere, and Lai Fu and Dan Dan are on the run. Peace on earth, good will to all?