Forget about what monstrosities we have done to other human civilizations throughout history. Instead, think about what we have done to animals. We have hunted many animals out of existence, or we have farmed them and made them basically the equivalent of the Matrix, sources of energy and food.
We are trying to eradicate mosquitoes for crying out loud, and entire species, without giving it a second thought. I will use insecticide to kill today entire colonies of ants without blinking.
All it takes is for one advanced alien civilisation to come across us and deem us the equivalent of their mosquitoes to eradicate us and take all the resources from the Earth. That's the whole point of the Dark Forest theory. If there's an infinite number of civilisations out there, and one of them is so advanced that we are insects to them, why wouldn't they just exterminate us, or use us as food?