Humans are violent because they are carnivores, they have a killer instinct. Even now people still get a buzz out of killing for fun. Like Trumps revolting murdering trophy hunting sons.
“As long as Man continues to be the ruthless destroyer of lower living beings, he will never know health or peace. For as long as men massacre animals, they will kill each other. Indeed, he who sows the seed of murder and pain cannot reap joy and love.”
― Pythagoras