Slavery

Today, I was reading up on Slavery in the United States. It had started in 1605 and went on for over 250 years. The slaves had no rights, civil or human. They were treated as objects owned by their masters, who used them in any manner they wished. I was horrified to learn that they could not even marry and their children, often fathered by white men were also turned into slaves based on their maternal lineage. Slaves worked day in and day out and their work stopped only on special holidays and Christmas. I cannot imagine how unlucky one was to be a slave and how cruel and unthinking one was to be a slave driver.
Then I remembered the ritual of "Sati", the practice of burning recently-widowed women, prevelant in Bengal and other states of the British-ruled India, till it was banned in 1832 as a result of the reform movements by Raja Rammohan Roy. He had written and distributed articles about how it was not a part of Hindu scriptures and brought awareness among common people.
How and why are people this cruel to their fellow people? How can differences in gender or skin color drive men to this unimaginable level of hatred and cruelty. I fail to understand the logical reasoning that went on in their minds that justified such actions. Are people so selfish that they don't understand the pain and suffering of those around them ? What is the use of our senses if we cannot feel for our fellow beings ? How can we be happy if we are putting others through utter misery ? I guess this is but human nature just like greed, anger and other failings. However, I still do not know the best way to deal with such negative behavior. How do we cure these people and teach them how to love ?