Here is a graph of American homicide rates, the earlier results should be taken with a grain of salt of course, but the trend is clear. N.B. These rates are per 100,000.
The American data is consistent with European data. Here is Table 2 from Manuel Eisner’s Long-Term Historical Trends in Violent Crime. Do note that some correction should be made for the fact that violence is less lethal when people are healthier and medical care is more effective.
The bottom line is that there has been a big and welcome decrease in homicide rates in Europe and America over the past several centuries. To put these numbers in perspective, however, note that the homicide rate in New Orleans today is 52 per 100,000 and in Detroit it’s 40 per 100,000 so even with a lower average there is lots of variation. Brazil today is around 22 per 100,000 not too far from America in the 19th century. The homicide rate in El Salvador is 71 per 100,000, in Jamaica (!) 60 per 100,000 and in Honduras 67 per 100,000 — all higher than fifteenth century Europe. Thus, the past was a more violent place but not so violent as to be unknown to the present.
Hat tip: Tim Harford.