If that is true WHY is there such a serious decline from 1980s and early 1990s to today in the States? Also why is there tremendous amount of police killings i

If that is true WHY is there such a serious decline from 1980s and early 1990s to today in the States? Also why is there tremendous amount of police killings in the 1920s-1930s is so far above from what they were in the 1950s, 2000s+?