The Killer Lives Next Door summary:

A killer who can completely conceal all traces of his existence has been hired to deal with the target while keeping a low profile. However, the place where the killer infiltrated was an apartment in Korea that was has poor soundproofing level. Everyone can hear everything that is happening in the apartment. It is in this place that the killer is to monitor the target until given further instructions. All he wants to do is to complete his mission in peace. However, things just won't go his way.