Nationwide protests over the Minneapolis police killing of George Floyd, the latest in a long string of high-profile slayings of black men by white officers, have featured a common rallying cry: “Defund the police.” The movement predates the current protests and is driven both by anger at the militarized posture of many U.S. police departments and by the recognition they are being called on to confront social ills including addiction, mental illness and homelessness that, advocates say, could be better addressed by spending on social services and rethinking what behaviors should be considered crimes. Demands on streets from New York...