Tyrant means a cruel and an oppressive ruler. “Sic semper tyrannis” is a Latin phrase that means "thus always to tyrants", i.e. “Thus always I bring death to tyrants".
“John Wikes Booth” was a supporter of slavery and the Confederacy. On the other hand, Abraham Lincoln was freeing slavery. He heard the President Lincoln's speech in which he summarized his plans to merge the South into the Union and to extend the franchise to some African-Americans. Booth believed that Lincoln would destroy the Constitution and his beloved South. He was angry with Lincoln even celebrations were taking place in the town on Union's victory. Thus, Booth shot Lincoln and screamed “Sic semper tyrannis” after he shot him, referring Lincoln as tyrant.
