Things happen for reasons. And fate is not itself a reason.
Even when Prophecy reigned, and gods meddled with the fate of mortals, the reason for many events was not "fate", but "a specific god did it". Or "a specific god did something that indirectly caused it to happen".
Even if it is his fate (he is only vaguely aware, in rare glimpses, of what his fate might be), it only happened due to specific events, choices, and random chance.

At the end, it doesn’t matter.
What matters is that Kaworu was in Westcrown on the night of Sarenith 3rd.

The right action was obvious.
Kaworu is not the kind of entity that looks for problems to fix. Who optimizes for a certain impact on the world.
But if something happens nearby, and he can directly help, he will help.

The true tragedy of the situation, Kaworu mused looking at the chaos, was not the violence itself.
It was that no one, no matter what side they consider themself belonging to, had reason to use violence. No one benefited from it. If no one did nothing, if everyone just continued their life as usual, it would have been better, for everyone. It’s what they would have done, if they had the insight and foresight – knowledge of the self and knowledge of the non-self (that is called "world"), the two components of wisdom.
But they don't. And so they act towards goals that do not exalt their souls. Or in other words, do not fulfill their true values.
The second part of the same tragedy, is that while some have a current goal such as "kill nobles at any cost", others have goals such as "survive", and would themselves prefer not to harm anyone. But they often do not have the ability to protect themselves from harm without harming anyone else.

[Moloch, the incomprehensible prison!]

How fortunate it is then, that those exist who learn to defeat an opponent, swiftly and successfully, without harming said opponent in any way.
Those like Kaworu Nagisa, master of the Merciful Art.

“Hello, good people!” Kaworu exclaimed.
He did it calmly and quietly, so maybe "exclaimed" is not the right word.
There are cases where talking calmly and quietly is not the best tactic. But Kaworu always does things his way, being Kaworu.
The armed mob noticed him anyway, as he was standing right in their way.
“Where are you going?”

"What do you care?"

“If your intentions are violent, you should forsake them. There exist evildoers in the world who would be good to kill. But you will not find them tonight, on your path.
If you merely wish for violence against those you hate, or profit, that is even more reason to go back. Many of you will die to violence as well, and in several days the survivors will be punished by the Crown”.

"Shut up and step aside!"

"I will not. For I hope to help you."

The result is obvious. They attack him.

Kaworu is not yet powerful enough to face dragons and hydras with ease (though he feels he will be, one day. Or should, in some way).
But mobs of commoners, or low level warriors, are not strong enough to pose any danger to him, no matter how numerous they might be.
(They might have, if he didn’t ask a 1st level wizard to cast Mage Armor on him, as a favor in return for his help, several minutes ago. Fortunate that Cheliax has so many low level wizards.)
While he is clearly powerful enough. But doesn't pose true danger to anyone, either.

Kaworu is not strong. His punches, if they can be called that, do not carry weight. And yet they are impossible to resist.

He can influence the ki circulation in their bodies to paralyse them, as most monks can.
He can influence their ki to make them temporarily blocked from the possibility of violence. A rarer technique, invented by Monks of the Lotus.

Both of those options would require exertions of his own ki.
They are not sustainable, not enough to face the whole mob.

But there is another thing Kaworu can do. Maybe magic, maybe divine blessing, maybe pure skill.
When he doesn't start the fight, doesn't want to start it, is attacked first, makes it apparent that he is not an enemy to those who consider themselves his enemies, and then demonstrates that he could win by force, but doesn't want to, chooses not to...they see it, and Understand.
Not mind control, but pure, transcendent communication. Souls touching lightly, through the seemingly impenetrable barrier between selves, for the briefest moment.

It doesn't always work.
Even when it does, and they understand Kaworu, they don't always agree with him, or do what he says. (Which is good, control never was and never should be his goal.) Deeply held beliefs are never changed, but it might help people making a rash decision or missing something obvious come to their senses, rethink the situation for a few moments. A little like the help of an Owl's Wisdom, or a similar blessing. Or a condensed telepathic conversation.

It is better to convince people, through words, or soul-connecting strikes.
But sometimes it doesn't work, or doesn't have time, and is forced to attack first.
That is still acceptable. The Merciful Art, enhanced by the blessing of Korada, incapacitates his opponents very efficiently. With luck, he can almost get one of them every second.
It is not even painful. You just feel weak, and not fully in control of your body, detached and a little floating. If you retain consciousness, that is.

After a time, everyone is either convinced (one way or another. But most aren't), or laying unconscious on the ground (not harmed, but Kaworu Stabilizes each one, just in case).
"You should all go back to your homes. Or try to help. Put out fires, if there are any. Help the wounded. Keep people way from dangerous places.
Not fight. Fighting will not help. Unless you are much stronger than anyone you will fight, or extensibility trained in ways to defeat your opponents without harming them. I do not expect either is true. You can come with me – if you keep your distances and stay safe. Or go on your own way. The choice is yours".

And then he continues onward through the city, seemingly unhurried despite the great speed at which he moves.