A ship only needs one hole in it to sink, the entire hull does not need to be missing. The body only needs one organ to stop working for the whole unit to die, the rest of it can be healthy and yet still die. Likewise a human being, even one who does good things in many ways, and who helps so many people, only needs one mortal sin without repentance to go down into hell.

We don't know where Mandela is now, because we don't know how he ended his life and what went on between him and God in his life. But we should never think that simply because a person is good in so many other ways, that this can excuse them for something evil. If I help a thousand people, and give all my time and effort at doing good in the world, but I torture and murder just one baby, and I am never sorry for it... in the end am I a good person or a bad person?