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
More testosterone = more aggressiveness. It's basic biology.