Mandela spent a lot of time in prison in South Africa. In 1964 he was convicted of of sabotage and conspiracy to violently overthrow the government, and sentenced to life imprisonment. This was called the Rivonia Trial. Mandela made a speech, said to be modeled on Castro's "History Will Absolve Me" speech, which you can read here.

Glenn Frankel at the Washington Post calls it The speech at Rivonia Trial that changed history

Business Insider quotes the Mandela Foundation to the effect that it held the court "spellbound for more than four hours".

It's a long speech.

In it, Mandela referenced Cato Manor and Sharpeville

 there was violence in 1959 when the people of Cato Manor protested against pass raids;On 21 March 1960, sixty-nine unarmed Africans died at Sharpeville.

Sharpeville was the massacre where white police fired on a mob of Africans; Cato Manor was where they didn't.

Sharpeville is the one you heard about in school, Cato Manor the one you didn't hear about.

Wikipedia says that

It is likely that the police were nervous as two months before the massacre nine constables had been murdered under similar circumstances at Cato Manor.

Writing about a similar massacre, committed by the current South African government just last year, I wrote

the Cato Manor Massacre, two months previous to Sharpeville, is almost unknown.That’s because the police didn’t shoot—and four white and five black policemen were killed and mutilated. Mutilated means, according to Wikipedia and other sources, with their genitals stuffed in their mouths. I am unable to find contemporary newspaper reports of that detail, because in 1960, you couldn't say "with their genitals stuffed in their mouths" in a newspaper. But you can bet the police at Sharpeville had heard of it.

It was that kind of activity that Mandela was imprisoned for conspiring about, and it was that kind of activity that he bravely refused to renounce. For example, in 1985, President Botha offered him conditional release from his life sentence , "conditional meaning that " that he renounces violence, and violent protest, as a means to bring about change in South Africa."

Renouncing the stuff with the genitals stuffed mouths, and renouncing "necklacing" where blacks accused of loyalty to the government and the rule of law were burned alive with tires around their bodies was apparently something Mandela couldn't do—on principle.

In power, as Steve Sailer and John Derbyshire said, he behaved a lot better than Robert Mugabe.  A lot better than most African rulers, in fact, although that's partly the "soft bigotry of of low expectations. In John Derbyshire's words

A black African leader who's not a crook or a thug — Mandela was a white liberal's wet dream.

De mortuis nil nisi bonum, though. Mandela had the opportunity to be vengeful, cruel, corrupt, and triumphally arrogant … and he passed it up. That's pretty good as human beings go. As Africa goes, it's sensational.

Yes, but let's not forget where he started.