"And if "sovereignity" somehow confers legitimacy, particularly in tyrannies, why was the Left so cranked about South Africa a few years back?"
Sovereignty doesn't confer virtue; it confers sovereignty. Note that, even though South Africa was engaged in a form of persecution that was not only murderous, but also generations old, and the Left, as you like to call liberal values, was cranked over it, and Democrats were in control of both Congress and the Presidency for part of that time, not once did the United States invade that nation. Other forms of pressure were used instead.
The system in South Africa was fought, without overthrowing its government and destroying its physical integrity.