Umkhonto, (or MK) the military wing of the ANC founded by Mandela certainly killed a lot of people, far more civilians including women and children than they did members of the security services. Their anthem Umkhonto we Siswe includes the line "We the members of the Umkhonto have pledged ourselves to kill them — kill the whites". So yes. Unquestionably Mandela was a terrorist.
Was he justified to take up arms ? Probably. The apartheid regime wasn't going to roll over and agree to democratic elections without some fairly vigorous encouragement.
Did it work ? Yes. He and FW de Klerc led South Africa out of the apartheid era without massive bloodshed.
The point about Mandela is not that he started out as a terrorist. It's the example of reconciliation that Mandela showed after his release which makes him so exceptional. Just a shame that his successors have let so much of his good work slip away.
Mandela could have been like Robert Mugabe. A terrorist who after a very brief interlude of pretending to be a democrat returned to being a terrorist, albeit in his capacity as President of his country.
