Well... keeps us on our toes doesn't it
That's always a good thing survival wise
I haven't actually studied the cause of those things nearly as much so it's a little harder to say, but frankly a trait where the person is prepared to do anything for their own happiness and survival sounds like something that might enjoy at least limited success evolutionarily speakng
And most likely it's within the standard deviation of our brain model.
No, it's just that back in the day they tended to become chief or emperor or some damn thing.
It was less of a detriment and more of a marketable skill.
Joking aside, there was an absolute SHIT tonne of unprovoked killings in the day. And no "police" to go after bad guys if they ran away. Some societies had better infrastructure for that than others but none were great as a rule. I mean seriously, what were the vikings other than people who went out and murdered folk for fun and profit?
And while there might be indications that people were being murdered it might actually be hard to say that it was a 'mass' murderer. Even today it's not like they kill every other day or once a week. So if two or three times a year someone goes missing, was it bandits? Romans? That peddler that just passed through last week? one of us? an animal? did they get lost?
And if it did happen, and they did catch the person, we'd never likely hear about it most of the time. Small communities like that did not keep much in the way of records, there was no 'newspaper' (or any paper for that matter).
Having said that, there are some recorded. I found this which might be interesting:
Serial killers are a special kind of evil. Since being coined by FBI investigator Robert Ressler, the term “serial killer” has come to denote any murderer
listverse.com