As I was discussing this with my partner we summarised this as:

Humans have always had the capacity for violence and murder; as populations grew, acts of violence could be larger, both in terms of number of combatants and also length of time of continuous fighting. This is a progression of:

• Small bands of people skirmishing with neighbours to
• Towns sending small raiding bands to
• Cities fielding an army for a summer campaign to
• Empires furnishing professional armies and sending them on multi-year campaigns, to
• Nation states using advanced logistics to maintain millions of soldiers in the field for years at a time.

Somewhere between city-states and full modern nation states, there have been full on campaigns of genocide. But genocide can be thought here definitionally as only possible with some significant number of people.

Unfortunately there is a deep dark part of the human psyche that has always been with us.