Actually, we did. Like most Americans, sadly, you know nothing of history beyond, say, 1980 or some such. If you did know some history, you would know ...
Like many people on Slashdot you seem to have a defective knowledge of history and the church.
If one were to look into the history they would find that you either grossly exagerate on these matters, or are simply wrong. Many of the early colonies were formed by religous sects coming from Europe. Once in America they adopted the European customs of institutionalizing the church with the government. Although in some colonies other sects were persecuted, few were killed. In any case it was nothing like the scale or severity of European persecution. Other colonies had different views. Rhode Island was formed with the ideal of religious tolerence, and other colonies were adopting laws for tolerance by 1650. Eventually all of the colonies adeopted the US Constitution, became states, and moved past that.
As to the "Christian justifications for the genocide against American Indians" I have to ask, what genocide are you referring to? There wasn't one.
Reject the Lie of White "Genocide" Against Native Americans
Were American Indians the Victims of Genocide?
As to your claims about "lines of Christian preachers submitted tons of briefs, all saying that their Christian God had deemed that black people were inherently inferior and not worthy of any basic human rights" in the case of Loving vs Virginia, which briefs are you referring to? The only brief I see listed from an organization claiming church affiliation was against Virginia's law.
LOVING v. VIRGINIA, 388 U.S. 1 (1967)
Briefs of amici curiae, urging reversal, were filed by William M. Lewers and William B. Ball for the National Catholic Conference for Interracial Justice et al.; [388 U.S. 1, 2] by Robert L. Carter and Andrew D. Weinberger for the National Association for the Advancement of Colored People, and by Jack Greenberg, James M. Nabrit III and Michael Meltsner for the N. A. A. C. P. Legal Defense & Educational Fund, Inc.
T. W. Bruton, Attorney General, and Ralph Moody, Deputy Attorney General, filed a brief for the State of North Carolina, as amicus curiae, urging affirmance
So it looks to me that your disparagement of Christians is based on what is essentially one half-truth and two whole lies.
Now that would be bad in and of itself, but you also overlook the many positive contributions made by Christians.
The abolition of slavery - Christian and churches drove the abolisionist movement. Perhaps you could start with this man:
William Wilberforce - the story told in this wonderful movie: Amazing Grace, released in 2007
Higher Education - Many of America's first colleges were formed by churches.
Health Care - Many hospitals have been founded by churches, or with church backing.
The Civil Right movement - Once again many churches were participants in the Civil Rights movement
There are many more that could be added to that.
Yeah, you Christians are really, really superior to other religions....
Moving past the half-truth and falsehoods you wrote certainly seems to make for a better record to reflect upon.