Murdering people was never common, certainly nowhere near as common as having sex. Don't forget that the histories you look at highlight the murders and wars just like modern-day journalism does - because it's "interesting and different". For example, the vikings are burned into our collective memory as vicious raiders - but they were traders that got on well with their neighbours. Murder has always been something out of the ordinary, right back to the bloke who jotted down "Thou shalt not kill"