"What does the history of the slave trade have to do with whether or not the Confederate flag is a sign of bigotry in today's society?"

Since the Confederacy was a system that relied on slavery to support it, the slave trade was essential to its survival. Since the Confederate flag was a symbol of the system, it had everything to do with it. Many of the descendants of the Confederacy still support the idea that slavery might have been justified because of their supposed notions of racial inferiority. The question posed suggests a complete ignorance of the feelings of contemporary African-Americans.