Moonlighting

In a quiet rural village, there's a place where laughter never ceases—from children to white-haired elders. It's the small piano academy run by the village's young headman. For reasons unknown, it's a one-man operation where a gaunt-faced man, referred to by the headman as his "wife," teaches all the students alone. Yet beneath this peaceful daily life lay a deadly twist...
As the academy closes its doors and night deepens, the village chief and the academy director begin gathering, one by one, the various murder weapons hidden in the academy's secret storage room.
A gang boss who abandoned his organization and went rogue, and the undercover cop monitoring him—that is their true identity, unknown to anyone in the village.
(Source: Lezhin, translated)