Malarkey
To agree with the author's premise is to concede the shooter's worldview of America's origins are factual. The shooter was upset that things weren't going well for him (or other white people around the world) and that things were better hundreds of years ago socio-politically. No matter the system of government, the shooter thinks it's better to have white supremacy and segregation (and even slavery).
A reading of the founding documents of this nation does not show evidence for the author's premise or the shooter's worldview.
All my life I grew up across the river from Mississippi and was told Mississippi is the most racist state. This author apparently wants South Carolina to take the lead.