Mandela was a terrorist (and so was George Washington). At the time of Mandela's arrest he and the ANC were promoting the use of violence to achieve political ends. That IS the definition of a terrorist; using violence to achieve political ends. If a commentator wishes to ague that the Sharpeville massacre was a terrorist act by an apartheid state, I would agree whole-heartedly. However, it becomes an interesting argument when one accepts the right of the occupied to fight back against the occupier.
Labelling non-violent protesters as "terrorists" is a step towards terrorism by government.