This document summarizes research on police ethics and analyzes the killing of Laquan McDonald by the Chicago Police Department. It discusses how police work involves discretion that can enable ethical lapses if not balanced with oversight. Police culture and the "code of silence" can encourage unethical behavior by rationalizing violence and protecting officers from accountability. The McDonald case shows how these issues manifested in Chicago through a history of misconduct, lack of reform, and an apparent cover-up by police and city officials of the circumstances of the killing.