So you would agree that the slave owners of 1776 were also not engaged in a legitimate rebellion against tyranny, right? Why was the Confederacy’s bid for self-determination any less valid than that of the British colonies? Both nascent nations embraced slavery.
The Colonialists of 1776 were engaged in a rebellion against tyranny. Whether or not their cause was legitimate is in the eye of the beholder (I am of the opinion that it was). The fact that many of them were slave owners is immaterial to their cause or their struggle. The fight in 1776 wasn’t about slavery - it was about representation and determination.
The south chose to institute war against it’s fellow states because it perceived a shift in the balance of power represented by the 1860 election. They feared that this shift in power would result in the abolishment of slavery. They had representation in congress and SCOTUS and had enjoyed “ownership” of the presidency in 8 out of the last 15 presidents (9 if you count the feckless buchanan).
Their cause was not just and certainly not legitimate and their course of action was ruinous.