Who gave these people the right to call their head of state a tyrant and not the people of the South?
The Americans in 1775 had no representation in Parliament and had no power or influence over the government that was exerting its authority over them. That is tyranny. In Southerners in 1860, by contrast, sent elected representatives to Congress and had their say in the presidential elections. As I already pointed out, due to the Three-Fifths Clause in the Constitution, Southerners actually had greater say in the government than did Northerners.
And while we are on the topic of legitimate revolution, did the slaves of the South have a moral right to rebel against the slave-owning class?