Even I am no expert on this and am wary of a lot of the information online as, even now, there are still strong feelings amongst some so it may not be impartial. From what I understand Mandela did not plant the bombs himself but was a leading figure in setting up the fighting wing of the ANC and, by his own admission, "signed off" approval for bombs to be planted to further their political cause.
If you Google Nelson Mandela bombings you will find numerous sites showing appalling pictures of the carnage caused by the ANC bombing campaign which killed and maimed many yet seems to have been erased from history. This is one . Here is another but, as I say, I am no expert and cannot vouch for the veracity.
I am a great admirer of Mandela and all that he has achieved since his release from prison but he wasn't always so saintly.