system
A chat on subreddit /r/NoStupidQuestions
human
The tendency to violence can be traced back to Evolution, men always had to kill (not saying women didnt had to, but men more so). 


I have heard someone say that thousand years of War brought a huge selective factor with it.
War is fought by men, as is hunting animals. The Man the most brutal has the highest Chance of survival, the man who kills survives.

The primal role of women did not require violence the way the men's did.
gpt
I rather disagree with the statement that violent men are selected for. Consider the men who were not on the battlefield such as farmers or blacksmiths who are required to support an army. In a conflict they were less likely to be killed because their skills were valued. Those that survived were more likely to reproduce than the men who died at war. Hence why the last name "Smith" is so common