I suspect the majority of white South Africans, certainly under Apartheid, considered Mandela a 'terrorist' - rather in the same way the occupying Nazis considered the French Resistance, the Maquis, terrorists during WW2. And whilst throw-away platitudes like, "one man's freedom-fighter is another man's terrorist" might settle the minds of those using them, they are, when taken in consideration, trite, empty and meaningless, and don't progress the discussion any.
By way of hypothetical example, had Herr Hitler been successful in invading and occupying these shores after the recent unpleasantness, both appeasers and collaborators would, I suspect, have been treated just as unforgivingly here as they were in post-War France. That, and I suspect the British people would have done what Churchill famously stated ("we shall fight them on the beaches..."), where the Germans would have found themselves in a 360-degree guerilla war, and the British people largely ungovernable. And I think we can agree that that would have required violence on a grand scale on our part.
Ever since the Norman Conquest of 1066, these islands and their peoples have not known what it is to be truly oppressed by a brutal, dictatorial regime which, by statute law, has made your very race inferior and only fit for subjugation. Mandela [who was, at this stage, a lawyer, and hence recognised unjust laws when he saw them] did know, all too well, and decided that enough was enough and opted to take up arms in the same way that we, as a people, would have done under any ensuing Nazi occupation - and it would be a fool who might suggest that the British people might have gone quietly into that good night without exacting murderous acts against any occupying force. Such was the situation in which Mandela, and the ANC, found themselves. Violence might be regrettable and (in calmer, more democratic times) hopefully avoidable; but there are times when only it will do, if you seek not to live under the yoke of racial [or any other form of] subjugation.
As for Messrs Adams and McGuinness: whilst these two are, without doubt, personally responsible for a slew of atrocious acts in the recent past, we are obliged to see these past acts for what they are - rungs on a ladder which has now led to a multi-party democracy taking root in Northern Ireland (or the north of Ireland, depending on your political bent); although that is not to detract from the nature of their previous crimes. In the aftermath of all wars, we must seek compromise: and The Good Friday Agreement was just such a compromise. It doesn't suit everyone, but it is better than the previous alternative. Any reasonable person would rather have them involved in the mechanisms of a plural democracy (i.e. the ballot box rather than the Armalite), as they are now, than still organising armed resistance/terrorism. As Churchill reminds us: "jaw jaw is better than war war".
It's also worth reminding ourselves that Mandela made it a staple of his post-incarceration, and latterly his presidential, years to seek reconciliation and forgiveness: when the broader part of his ANC wanted, for example, to prevent the whites from playing Rugby (the one sport which every whiteman in South Africa held in the highest regard), and if not prevent, then ensure that they could not use the Apartheid-era name, The Springboks. Mandela refused, as he knew that would just throw fuel on the fire and give the AWB whites just another reason to set themselves up for a civil war. He famously wore the Springboks' captain's, Francois Pienaar's, number 6 jersey at the Rugby World Cup final against the All Blacks in 1995 - that act alone accorded South Africa a large element of healing past hatreds. Prior to that, South African blacks had, routinely, supported any other team playing the Springboks. After that, all South Africans supported the 'Boks.
In truth, we can debate Mandela's legacy all day, but his country and its people are, by and large, far better off under the plural democracy which he instilled than they ever were under Apartheid. We also have to agree that no democracy is perfect but, as Churchill again reminds us, "Democracy is the worst form of government, except for all those other forms that have been tried from time to time."