@Dr Sliptinschit,
First... let me state that I am a strong supporter of Free Speech. I believe that White Supremacists represent the worst form of bigotry. However I will defend their right to express their opinions. That being said...
White Supremacy is the idea that the White race is superior to other races. They believe that Black people and Hispanic people are, by nature, less intelligent more prone to crime than people of other races.
The consequence of White Supremacist beliefs is a support for inequality. They believe that White people should be especially protected against other races. They believe that White people should have the majority of the political power, and that White cultural practices should dictate the culture of the United States.
The question "what's wrong with this" is a clearly subjective question... but I believe that my answer speaks to the
core of values held by most Americans.
We Americans, as a people, believe in equality, and fairness, and freedom. Our founding documents said "All men are created equal and are endowed by their creator with certain inalienable rights...". Most Americans believe in this. Now, yes, we started as a country with slavery, and we fought a civil war and we are still in a long struggle for civil rights... but we have been moving toward these basic values for hundreds of years now.
White Supremacy rejects these core American values, by painting America as a White Christian nation to the exclusion of all of the other people who live here. White Supremacy means inequality... it means that whether you are treated well by police, or treated with suspicion by neighbors, or treated fairly by employers and banks, depends not on who you are, but on the color of your skin.
As an American I see this as deeply unjust, unfair and against the fundamental values on which our country is based.
That is why White Supremacy is a bad thing.