It depends on what your opinion of the worst case scenario is I think. Nuclear war is probably the undisputed worst possible outcome, but there'd lots of lesser, more diverse flavors of awfulness. On the one hand there's the possibility of increased racial violence which leads to a crackdown by the Apartheid government on dissent. Eventually that boils over into full blown racial warfare and the country, insofar as it exists, becomes a highly militarized state flush with foreign supplied weapons. On the other hand the government could be toppled, and the same situation could develop, but with less cohesion between armed groups like the remnants of the SADF. You could end up with regiments and divisions fighting to secure positions and purge anyone they deem as opposition. The same with the armed African nationalist groups. In either situation the global community is likely to step in, particularly if the government falls and is unable to keep a firm grasp on its nuclear weapons.
Maybe the worst outcome in terms of human casualties is the possibility of the development and deployment, intentional or accidental, of a targeted ethnic weapon. I've read that under programs like Project Coast the government investigated the possibiliity of sterilizing or killing targeted populations through biological weapons. Such a weapon is probably not even possible, but if it were, and South Africa created it, things could get VERY ugly. I don't know enough about biology and virology to suggest a potential POD for creating such a device, but maybe someone else has a suggestion.