I have nothing to say that others wouldn't have covered much better on the meaning of Nelson Mandela in the history of South Africa and the world. But I would like to quote from Adam Serwer's article:
The point of remembering all this is not mere point-scoring. It is to remember that sometimes the radicals are correct, that in the heat of the moment, movements for justice can be easily caricatured by those with authority as threats to public safety, and those seeking basic rights and dignity as monstrous villains. And then after the radicals win, we try to make them safe and useless to future radicals by pretending our beloved secular saints were never radical at all.
It’s tempting to pretend we’ve all always agreed about Mandela, or about racial equality, or about South African apartheid. It would avoid awkwardness or hostility to join together in mutual admiration and mourning for a figure who was indispensible in so many senses of the word, without recalling those who stood against him.
Mandela believed in forgiveness, but he also believed in truth and reconciliation. And the truth is that many self-proclaimed champions of individual freedom in the United States refused to champion the individual freedom of black people in South Africa and at home.
Without intending to draw any comparisons between the South African apartheid and other types of societal ills, I believe that the treatment of past revolutionaries of different types does tend to follow that pattern when the revolutionaries win. What they accomplished will be absorbed and tamed and it is that version which will be added to the general history of a country.*
I am not sure if this kind of sanctification of taming isn't just the way human beings digest the changes that take place. But Serwer has a point in the case of Mandela and also Martin Luther King, because the process of society-wide sanctification has been very rapid for both those men.
*Hence the long struggle of women's suffrage in the US became "women were given the vote" and Susan B. Anthony, Elizabeth Cady Stanton, Lucretia Mott and Sojourner Truth are now seen as worthy of statues.