Reagan in the end allowed Mandela to become Mandela. If the South African economy was wrecked by sanctions, the ANC would have followed the road of Mugabe. However, because of Reagan, South Africa had a vibrant economy at the transition. Mandela and the ANC saw themselves becoming the hegemonic power in Africa because of South Africa’s economic and military power. They needed the whites to achieve this goal of dominating the continent. This desire to project their power outwards required cooperation at home. Reagan allowed Mandela to see himself as a world leader instead of a petty local tyrant.
But his veto was overridden and the sanctions went in anyway. On most of these Mandela threads the consensus seems to have been that it was the sanctions that ended apartheid and not Mandela, and that's probably accurate. So then if the veto had not been overridden and sanctions were not enacted then wouldn't that mean that either apartheid would still be in place or the ANC would have resorted to ever bloodier terrorist acts? Or both?