Will oppressed people ultimately rebel nonviolently or violently against their oppressed, as King says they will?

King's assessment was accurate. Historically, those who've been oppressed will ultimately strike back at their oppressors. Rebellions, wars and strikes are examples of this discontent. 

The Negro has many pent up resentments and latent frustrations, and he must release them. So let him march; let him make prayer pilgrimages to the city hall; let him go on freedom rides -and try to understand why he must do so. If his repressed emotions are not released in nonviolent ways, they will seek expression through violence; this is not a threat but a fact of history. So I have not said to my people: "Get rid of your discontent." Rather, I have tried to say that this normal and healthy discontent can be channeled into the creative outlet of nonviolent direct action.