The problem with your theory is that in the Middle ages, the homicide rates in Europe were similar to what we see in urban ghettos today.
It is not genetic, but cultural; principally the lack of trust in a criminal justice system.
Forget what you were taught about this in school. Research current publications on violence linked to genetics. Keep an open mind while you do this, understanding that every possible effort has been expended by those who would like to deny this linkage.