It also has to do with the fact that local communities had a significant population in comparison to the native american m tribes who lived in the area.
It's a good we got out in front of that issue ahead of time by killing Native Americans and then moving them away from their ancestral homeland.
credit where credit is due - I blame the British empire. NZ, Australia and other former colonies have done it as well, getting slightly less vicious and slight more manipulative with time. Dodgy treaties with locals etc.