Seems like a silly question. If everything is just then injustice doesn’t exist. Just like if everything were yellow then red wouldn’t exist.
But in his Meditations, Marcus Aurelius writes,
If you attend well, you will find that whatever happens, happens justly. I don’t mean only in an exact order and destined connexion, but also according to justice, and from one who distributes according to merit. Go on in observing this, as you have begun: and whatever you do, do it so as you may still remain good, according to the intellectual and true notion of goodness. Observe this in all your actions.
This could be used to justify terrible things. Is every murder and rape in accord with justice and distributed by merit?
Such attitudes were common in ancient times, though. Elsewhere, Marcus says we should welcome whatever happens to us, even if it’s cruel, because it’s for the benefit of the universe (5.8).
His viewpoint is consistent with the ancient Stoic belief in Logos – that divine reason orders the universe, that it’s providential, and what follows from it is our fate.
In 1759 Voltaire made fun of the idea that everything is for the best in his short book Candide. Despite a series of tragedies, Dr. Pangloss comically denies reality and remains childishly optimistic. Before the movie Pollyanna, a naive person with rose colored glasses was called Pangloss.
Many modern Stoics don’t agree with Logos in the ancient sense. But this greatly alters Stoicism. No longer is the universe ruled by reason, though the exercise of reason is still a human ability that is central to Stoic philosophy.
And there is no providence. In my view, if Logos is even a useful concept in the modern world it must be brought in line with science. The laws of nature, as we currently understand them, follow from four forces of nature: the strong and weak nuclear forces, the gravitational force, and electromagnetism. Physicists are searching for a “theory of everything” that will bring these together to reveal the underlying principle of the universe.
But this will not be a conscious, providential principle like Logos. Instead, the universe is indifferent to our existence and is unaware of whether we suffer or not.
Natural disasters, then, are not unjust because there’s no intention behind it. Justice and injustice are human products, the result of the actions of billions of people. And this is where Stoicism is still relevant. The only thing one controls is one’s deliberate thoughts and opinions, and virtuous actions can only follow from virtuous thoughts and opinions. So everyone has a responsibility to contribute to justice and avoid injustice.