Isn't it strange how until quite recently homo sapiens found it unnecessary to create massive justice systems? Logically, it is hard to argue that our biology has changed more in a thousand years than in the 99,000 ones preceding them. Is it perhaps reasonable to consider that our present societies themselves have a negative impact on human beings who otherwise would be peaceful and cooperative (unless threatened, of course)? Anyone who has raised a pet or a child---- or been much around others' pets and kids--- knows you can seriously alter a personality by how you treat it. Such a person also knows that individuals come "out of the box" with certain personality characteristics, such as high aggressiveness. 
Put those together and weigh them against a complete "free-will" attitude.