Do you have a source? Mandela himself was pretty clear on violence being a tool to effect change.

In his own words:

Four forms of violence were possible. There is sabotage, there is guerrilla warfare, there is terrorism, and there is open revolution. We chose to adopt the first. Sabotage did not involve loss of life, and it offered the best hope for future race relations.

http://www.theguardian.com/world/2007/apr/23/nelsonmandela

The entire speech is worth reading.