Mandela was an imperfect hero. How many perfect heroes are there? What he did saved South Africa from becoming Zimbabwe. It may still get there, but that would be the work of his successors who did not learn the lessons that he did.
Mandela was no saint to be sure, but nor was he the only party in SA with bloody hands.
Heh. Try being a white landowner in South Africa and then tell me all about how wonderful and non-Zimbabwe-like it is (presuming you haven't been set on fire yet). It's the same thing in slow motion. The end of Arpartied is leading, as feared by the white minority at the time, directly to genocide. The pogrom is happening now, and it's a direct result of one Nelson Mandela.
I could go on about how Mandela himself was, but at the moment the world is in the "OMG, he walked among us!" swoon phase, and observations of his being an unrepentant terrorist will be lost on people. The fact that his wife Winnie was one of the most vicious murderers to walk the earth will be lost as well.