How did john brown's raid frighten many southerners?

The main way in which John Brown's Raid frighten many Southerners is that it showed that massive slave uprisings were a possibility. Although his mission ultimately failed--it showed that slaves were willing to take up arms against their captors.

John Brown's Raid was organized by abolitionists who supported freeing the African-American slaves. This frightened the southerners because they believed that such a rebellion was not possible as slaves wouldn't dare to fight against their owners, let alone with guns or similar weaponry. This is why they feared since they believed this might inspire other slaves to try the same.