> Oppression would be quite impossible throughout history if people weren't willing to oppress their own kind to the benefit of others.

No what's far more common is that people change their perception (or have different perceptions) of who is "their own kind."

You can actually see this happening in real time in the US with the emerging concept of "Heritage Americans." It's a way for losers and crybabies to narrow the scope of who is "their own kind" without having to openly declare that they simply don't love their countrymen.