John Wilkes Booth shouted "Sic semper tyrannis" ("thus always to tyrants") after he assassinated President Abraham Lincoln on April 14, 1865. Booth believed that Lincoln was a tyrant who had overstepped his authority, particularly in the context of the Civil War and the treatment of the Confederate States. He was a strong supporter of the Confederacy and believed that Lincoln's actions during the War, including measures such as the suspension of habeas corpus and the Emancipation Proclamation, were dictatorial and oppressive.
Therefore, the most accurate answer to why Booth shouted this phrase is: He thought both the Confederate leaders and Lincoln were dictators.