Spanish and Portuguese colonizers were much more ruthless towards Native Americans during the Age of Imperialism, so maybe we should be glad that UK eventually dominated during those years.
And in US history books, they teach you about slavery, the injustice towards Native Americans, and their fatal mistakes in Vietname War over and over again from middle school all the way till college. US even pays compensation money to those people who were oppressed by its past government. There really aren't many governments that can measure up to what the US has.
The founding fathers of America were really onto something when they wrote the Constitution and set up all those checks and balances.