"They didn't have a right to secede, which is why it's called the "Revolutionary War". They knew that their actions weren't legal and that they would have to fight."
Indeed. Which is why they wrote the Declaration of Secession, er, Independence, in which they said that the right to liberty comes NOT from governments (which will almost always try to deter people from freedom), but from God Almighty.
"And what acts of tyranny were the southern states subject to?"
As stated, when governments use FORCE to FORCE themselves upon those who wish indepedence, there is no clearer sign that tyranny is present. Such as King George, Lincoln, Chairman Mao, Krushchev, etc, etc.
Ironically, some years ago, you may remember that President Clinton visited Communist China. During his visit, Clinton asked what happened to the famous "student" in Tienamin (sp) Square who stood directly in the path of the tank, that was immortalized in news reels across the world. After responding that they were not sure what happened to the man, one of Mao's assistants told Clinton:
"We acted precisely as your President Lincoln."
Indeed.
It is no small irony that one of Lincoln's biggest fans was a man by the name of Karl Marx.
Bullshit China story aside, the question still remains, what tyranny were the southern states subject to? They had representation in Congress, a voice in national elections, the freedom to run their states pretty much the way that they wanted. It's not like the Founding Father's who had zero say in the government. On the contrary, the southern states had exercised a disproportionate level of influence on national policy since the adoption of the Constitution. The only 'tyranny' seems to be the 'tyranny' of the majority. I don't think that the Founding Father's had that in mind as justification for rebellion.