People have been coming up with theories about this forever, from perspectives and time periods as diverse as Aristotle, St. Augustine, Gandhi, and Trotsky. You put a lot of very difficult questions in your post, but you didn’t put forth a criteria for what “justified” means to you. I think you’re going to need to interrogate that before being able to even think about any of these questions. For example, is violence justified by better outcomes, or by some absolute individual right to fight your oppressor? Is justification a question of morality, legality, tactical value, or something entirely different?