Anthropologist Peter Frost and anthropologist and population geneticist Henry Harpending argue that killing murderers pacified the population eugenically.

At the beginning of [1500]… the English homicide rate was about 20 to 40 per year per 100,000 people. At the end [1750, AT], it was about 2 to 4 per 100,000, i.e., a 10-fold reduction (Eisner, 2001).

…Can this leftward shift be explained by the high execution rate between 1500 and 1750? During that period, 0.5 to 1% of all men were removed from each generation through court-ordered executions and a comparable proportion through extrajudicial executions, i.e., deaths of offenders at the scene of the crime or in prison while awaiting trial. The total execution rate was thus somewhere between 1 and 2%. These men were permanently removed from the population, as was the heritable component of their propensity for homicide. If we assume a standard normal distribution in the male population, the most violent 1 to 2% should form a right-hand “tail” that begins 2.33–2.05 SD to the right of the mean propensity for homicide. If we eliminate this right-hand tail and leave only the other 98-99% to survive and reproduce, we have a selection differential of 0.027 to 0.049 SD per generation.

…The reader can see that this selection differential, which we derived from the execution rate, is at most a little over half the selection differential of 0.08 SD per generation that we derived from the historical decline in the homicide rate.