The evil in civilization does not preclude evil among noncivilized people. The fact is that slavery was not nonexistent among forager groups. They could have used them, for example, to do the 'dirty work' for the group (there being always work that is the most undesirable) or, say, to test untried plants for toxicity. The fact is that uncivilized people were capable of engaging in torture of 'others', including using it for entertainment--much as 'testing' for witchery and burning alive those found 'guilty' was a form of public entertainment in the Middle Ages.

Mistreating 'others' was not a new thing introduced by sedentary people--though 'other'ness within the group (a society), to include economic class, gender, and (as societies became more complex) beliefs, was a new thing. (I define 'other'ness as the position that people who are different are not worthy of being taken into consideration in a way that people who are the same are.)