The screaming tabloid headlines harken a darker, scarier time in New York history. “Four more murders in one night,” the New York Post wrote under the grieving loved ones of a shooting victim. The front page of The Daily News blared “Subway Crime Soars.” In reality, the uptick in violence is small. But even the perception that New York is suffering any sort of backslide into the violence-plagued bad old days of the 1970s has empowered critics of Mayor Bill de Blasio. De Blasio has assured the public that overall crime is down and that measures are already being put...