Blacks, whites, every race welcome?

Why is it that today, we, generation's later use the sins and mistreatments of our ancestors to justify arguments of today. Some of my families gene contribution has all been wiped out. Indian and black being the two main ones. I am considered white and so are my family members because through out the generations we have had many different family members. We began Irish and French until we got here. So in saying all of this back to my question. I want us all to remember our heratige and learn about the atrocities that have occured. If our history is forgotten we are doomed to repeat our mistakes. I just do not see the sins or mistreatments of our ancestors being used as any justification for racism, or hatred, or arguments used today. What are your opinions? If I am not understanding tell me something that will help me to understand. Any one being hateful will be reported. Not that racists care but the threat is there anyway.