He is a former psychiatric patient with severe schizophrenia. Once he was released (or he escaped). He began killing residents of a high-rise building, hiding in an elevator shaft and literally becoming his victims: he occupied their apartments, used their belongings, answered calls. The police couldn't find him for a long time because he lived "inside the walls." As a result, he was neutralized not by the police, but by two residents (in the good ending, you + grandma).