If you advocate violence, you are not nonviolent, even if you don't personally use the gun or the knife. Supporters of the IRA, supporters of Hamas, supporters of Benjamin Netanyahu, Turning Point USA, none of them are nonviolent and none of them can claim the mantle of non-violence.
When Martin Luther King Jr's Southern Christian Leadership Conference decided to undertake nonviolent civil disobedience, they eschewed even violence in self-defense. They knew that any acts of self-defense would be spun as offensive violence.
Self-defense itself can become an excuse for offensive violence. Some of the most violent movements claim to be defending themselves: Nazis claim to be defending themselves against Jews, US white nationalists claim to be defending themselves against black people, and so on.