Master Fu doesn't talk about what destroyed the order. He merely tells Marinette it was a terrible mistake, that he was young.
He doesn't tell her what the mistake was, or why he murdered all of his fellows. Why he burned the place to the ground, did his best to efface its very memory.
Master Fu is 186 years old, and Marinette has never asked, why is it that the world has seen such transformation in such a short time. Why, in all the centuries, when an entire order of Miraculous wielders existed, there were so many terrible governments.
The answer is simple. Mages do not create out of the goodness of their hearts. The kwamis were betrayed. And for thousands of years, the order used their power. Peasant rebellions failed in the face of reality warping powers, and the great kings and emperors of hte world paid handsomely to obtain such power—and those who didn't, were made examples of. The Mandate of Heaven, far from being a symbol, was real—dynasties rose and fell based on whether or not they maintained the favor of the order.
And not just China. Scientists have come up with theories, but none of them quite explain how a plague could be so virulent that it would nearly depopulate the Americas, leaving them open to conquest.
But the Order could explain it. Disease, after all, is a concept, and one that filled their treasuries with gold. And so the order continued, prospering, striking down groups that might challenge them, molding the earth, and becoming ever richer, not just in gold, but in forbidden knowledge, secrets and favors.
There is a reason, why, even today, the idea of all controlling secret conspiracies seems to be baked into mankind's psyche.
And, looking upon the burgeoning empires of mankind in the 1850s, the Order was prepared to once again unleash their power openly, to cast mankind back down, so that once again, leaders would be desperate for any advantage, and none would think to challenge the Order.
But Master Fu, the newest initiate, was horrified. Horrified enough to conceal his true motivations, to steal into the order's vaults and unleash magic that in a single day and night of purifying flame, scoured them from the face of the world. He took the indestructible kwami, and fled, seeking to be a guardian in truth—to protect the kwami from those who would wield them…
And the world from the kwami.
And without the order, without the blades and sorcery in the night, mankind has moved on. Industry and science flourished, and less pleasantly, so has war. A world that was growing more complex even in his youth has exploded.
Science, technology, a man on the moon—all indirectly the result of Master Fu's decision.
But at night, he also remembers that Stalin, the Holocaust, a thousand wars, are equally his doing. What the Order did was worse, but Master Fu does not seek to dodge his responsibility.
And so, the man with enough power to master the earth lives humbly, in a small shop in Paris, using only the money he has earned with his own work. There is enough gold in the ruins of the Order's monastery to make him wealthy beyond measure, but it is soaked in blood. Fu will not touch it.
He seeks to bring the last two Miraculouses back into his possession to make them safe. But he has another goal.
Marinette does not know it, and Fu regrets it, but her test is not whether or not she will be a good ladybug, but whether or not she will be strong enough to one day become the custodian of powers that could make her an empress—and never use them. Never use them despite the hundred injustices she sees, or the personal sorrows in her own life.
Never give in to the siren song of power that the Order so eagerly embraced. To spend her life bearing a responsibility that would crush others.
Master Fu regrets it, but the Miraculouses are indestructible.
And he is not.
So now, at the sunset of his life, he has no choice but to find another to bear that terrible load.
Have pity for Marinette.