Where do atheists get their morality from?

[Boccaccio]

But genocide promotes your own survival by eradicating competition for limited resources.

So how did genocide become evil?

Have you read anything about serial strategies in game play?

It became evil when we turned to agriculture and city states, realising that if genocide were acceptable then one might well find oneself the genocidee rather than the genocider.

Optimal strategy in the longer term for ourselves and our descendants is not to kill, nor to engage in genocide, but to co-operate in managing resources and access to them on a relatively equitable basis. Genocide was the tribal religious practice celebrated still by people like W L Craig (Those poor soldiers, feeling bad about killing everyone on god's command. Have a bit of sympathy!)