What native tribes did to one another was still nothing compared to the scale of what Europeans did to them.

And one doesn't justify the other. If I break into your house, kill your family and steal everything not nailed down, including your youngest kid who I raise as my own, and the bones of your dead aunt that I put in a window display, I can't justify that by pointing out that you got into a fight with a kid in high school one time and once stole a pair of sunglasses from the Wal-Mart.