Slain Police Officer Wenjian Liu believed in leading a life of service — and his dedication was evident on the day he was killed: He volunteered to work a fill-in shift when a fellow officer was late. Liu’s selfless act meant that he died alongside Officer Rafael Ramos when a crazed gunman opened fire on their patrol car in Bedford-Stuyvesant on Saturday, executing the pair simply because they were cops.“This is a difficult time for both of our families,” Liu’s devastated widow, Pei Xia Chen, told reporters.