It would be more accurate to say that the book is a history of intentional killing in America, though I can understand why author and publisher would prefer a jazzier title. He also takes into account police killings, casualties in riots and labor disputes, massacres of Indian tribes, vigilante killings, and the work of lynch mobs. Statistics on homicides, especially historical statistics, are weak. Records, where they exist, are fairly unreliable. Obviously, Lane's conception of the subject muddies these waters even more.
Killers are of every race, and every station in life.
The modal killing comes out of this feisty cohort, and originates in some brawl, incident, or fight that turns lethal. The same holds for most domestic murders. Yet homicide rates have gone up and down. A new surge started around 1960, and it more than doubled the homicide rate. Since the 1970's, the rate has been fairly flat, but on a rather high plateau.