If you're strongly opposed to all violence, you practice pacificism, the philosophy that war and brutality are always wrong.
During wartime, someone with a strong belief in pacificism might participate in protests and marches, or possibly register as a conscientious objector, officially declaring their pacifism and their right to refuse to serve in the military. Pacificism comes from pacific, "tending to make peace," which is rooted in the Latin roots pax, "peace," and facere, "to make."