As Dave says above there is an assumption that 'they are lower than us - it's ok for us to treat them badly'

Give any group of people a collective dehumanizing name and you have the basis for torture, genocide etc from early recorded history to Abu Ghraib.

Perhaps one of the most infamous is the treatment of the Tasmanian natives by European settlers and escaped convicts.

'As early as 1804 the British began to slaughter, kidnap and enslave the Black people of Tasmania. The colonial government itself was not even inclined to consider the aboriginal Tasmanians as full human beings, and scholars began to discuss civilization as a unilinear process with White people at the top and Black people at the bottom. To the Europeans of Tasmania the Blacks were an entity fit only to be exploited in the most sadistic of manners--a sadism that staggers the imagination and violates all human morality. As UCLA professor, Jared Diamond, recorded:

"Tactics for hunting down Tasmanians included riding out on horseback to shoot them, setting out steel traps to catch them, and putting out poison flour where they might find and eat it. Sheperds cut off the penis and testicles of aboriginal men, to watch the men run a few yards before dying. At a hill christened Mount Victory, settlers slaughtered 30 Tasmanians and threw their bodies over a cliff. One party of police killed 70 Tasmanians and dashed out the children's brains." '

from BLACK WAR
THE DESTRUCTION OF THE TASMANIAN ABORIGINES

By RUNOKO RASHIDI