The killer in "The Residence" was Lilly Schumacher, the White House social secretary. She murdered A.B. Wynter, the White House's longtime chief usher, during a state dinner. Lilly poisoned Wynter with plant fertilizer and then bludgeoned him with a large clock to finish the murder. She had motives including Wynter threatening to expose her shady financial dealings and mistreatment of colleagues. Detective Cordelia Cupp ultimately uncovered Lilly as the culprit after piecing together the events of the night and noticing key clues such as a sealed door hiding the murder weapon