Paradoxically, the goal of supporting Mandela and the ANC in military training was exactly the opposite of what one might think. By supporting and encouraging military and armed resistance, including sabotage and assassinations Israel's goal was to make sure that the ANC was actually 'tainted' and discredited in the eyes of the world opinion. At these times, it was clear that the Apartheid regime was bound to collapse one day or the other as world public opinion was more and more aware of their abuses. For Israel it was critical to ensure the survival of one of their few allies and they calculated that supporting an armed struggle would actually be more beneficial for the South African regime. Pragmatism is probably what drove Mandela to actually get a training from a regime which was actually acting against their own ideals.
Ya'alon met with security chiefs to assess situation following developments at Gaza border (Haaretz)
from the article: Mandela received weapons training from Mossad agents in Ethiopia