The US, Australia and my home country South Africa were all founded on genocide, racism and slavery. In reality it really just boils down to how things were done back then. Until, relatively recently slavery was legal and supported in pretty much the whole world. To my knowledge you can still find a country or two where slavery is legal, not to mention the illegal slave trade that happens today. Human history is covered in blood, no amount of being politically correct will change that. We must accept our origins and live the best we can.
I wouldn't be surprised if in 100 years our culture is mocked as backwards and evil for how we treat some group of people or each other. Similarly, if we ever manage to get space flight down, I can see us taking over some aliens planet through exactly the same means, genocide and slavery.