According to mandela, why did he and others make the decision to form the organization umkhonto we sizwe?

B/c first they believed that as a result of Government policy, violence by the African people had become inevitable, and that unless responsible leadership was given, there would be outbreaks of terrorism. Secondly, they felt that without violence there would be no way open to the African people to succeed in their struggle against the principle of white supremacy. 

This is mentioned in his book, "I'm prepared to die"
Hope this helps (: