John Wilkes Booth shouted "Sic semper tyrannis" after assassinating President Abraham Lincoln because he viewed Lincoln as a tyrant. Booth was a strong supporter of the Confederacy and believed that Lincoln's leadership was oppressive and dictatorial, particularly regarding his actions during the Civil War. By exclaiming this phrase, which translates to "thus always to tyrants," Booth expressed his belief that tyrants should meet a violent end. In his distorted view, Booth saw himself as a liberator of the South, taking a dramatic action against what he perceived as a tyrannical government. Therefore, the best answer to the question is:
He thought both the Confederate leaders and Lincoln were dictators.