A Stoic Seeks Justice in Himself
The self is first person you can fix

Live in accord with Nature. Everything comes backs to this. If something is unjust, it is due to a conflict with Nature, but Nature is not unjust. Nature simply is. It is our context for making choices and nothing more. When something is unjust, ask then what you might do better. Ask what you might have done better.
You are not surprised when a bear mauls you, for intruding on her den. Why then are you surprised that a trooper stops you, for speeding on his highway? Was he being too hard on you, or could you have departed earlier and driven safely? A beast mutilates you, and you understand why, but one traffic ticket confounds you. What does this say about your expectations? Are they realistic? Are they in accord with Nature?
When you take up a cause, ask what you might have done to create the problem. Did you litter? Did you steal? Did you give money to people who would misuse it? Were you always honest? Did you drink and smoke during the election? Protest away. Resist tyrants where they rear their ugly heads. Remember, however, that the free speech of hypocrites will fall on deaf ears.
Perhaps, instead, you are offended by the activists. Perhaps you wish they would shape up and accept responsibility. Teach them their errors where you see them, and do so with grace, but grace requires the same humility. Did you enable the problem that the activists resist? Are you upon an ivory tower, blind to your own failures? Could you have paid your workers better? Could you have made safer products? Have you honored all your contracts? Did you bully lawmakers who do not represent you? Are you surprised that the People protest you?
Whichever applies, take up the cause against injustice, and understand that the first person you need to educate is yourself. Align your expectations with the facts. Seek accordance with Nature of all forms, of the wilderness, of the cosmos, and of human beings. It is not that you are evil because you are evil. You are merely evil because you are unwise. With wisdom will come virtue and thus the tools for a just world. This starts with seeking justice in the self.