I mean, he used to blow up buses. The point is to get attention and force the powerful to address the injustices.
Those 27 years in Robben Island did something to him. And the movement continued, changed, deepened, grew. They helped sing him out of prison: https://www.youtube.com/watch?v=XKCk8o5xzaM
He came out and helped to lead a nonviolent transformation (of people, the state, the society). That was the method. The point was still to force the powerful to address the injustices. Rivonia Trial speech: http://www.rfksafilm.org/html/speeches/mandela.php
Some people say that you have to be transformed and all peaceful internally before you do something powerful in the world. But I think Nelson started before he was “all together” internally, and it was the process of healing his world that he healed himself.