Stress, Hate and Anger.

What happened with Sao was definitely the last drop. He always obeyed him and was totally faithful to him, yet he prefers the three of them. He treats them like his golden children compared to how he treats him and what really makes him pissed off beyond measure, in the whole matter, is how disrespectful Siwang is, towards him. And he is really not a patient and forgiving man.

When he left, during the dinner, he didn't go home, like Sao thought he would do, knowing him. He went straight to the IA's hq, taking profit of the fact his cousin was busy and wouldn't head back until he would return to the North.
He entered the Captain's office and searched among the future missions he had noted. He picks one, in particular, and writes a duty order, changing the target of it. He uses Sao's ring to mark the orders as official and put the parchment in his uniform, after leaving everything in the exact same spot as they were before he came.

With that done, he simply returns to the North and waits a couple of days, during which he really keeps himself on a low profile, staying by himself all the time.
When he feels the moment is the right one, he sends the fake orders to Siwang, through magic mail.
The target of the mission is Ding Ju, a powerful and righteous Master of Martial Arts, whom lives in a very isolated place with his son and all of his skillful students. The accusation is treason and he should be taken into custody for an interrogation.

Nothing shows that it is a fake orders, so the three IA prepare and head to the remote place where the mission leads them to go. It is a big thing, a success in that would bring very big credits for all of them.
Once reached the school, the IAs are let inside and try to convince the suspect to hand himself willingly, in the promise of a privileged treatment.
The Master and his students are outraged by their accusation. They know what IAs methods are and that they would torture a fake confession out of him, unless he accepted the accusations.
A very rough fight begins, where the 3 are hugely outnumbered. As they try to reach the gate to retreat, though, they find out it was blocked from the outside and they have no way to escape without committing a genocide.

The fight lasts for a long time and despite the odds, all against them, the 3 IAs manage to kill all of the opponents, leaving a cemetery of mutilated corpses all around. They were forced to kill even the Master and the only survivor is the son, a young man who simply give himself in, when he realized what massacre they made.

They have to climb over the gate in order to leave the place and when out they realize that the door was chained. The young Ju swears it could not be the doing of any of his dead comrades and that he had never seen those chains before.
They call Eizan to escort the prisoner to the hq as they can not approach the capital city, and still totally covered in blood, they head back to KM's home, exausted and tormented by mixed thoughts.

It was Zhan to chain the gate to trap the three inside the school. He wants them dead and he can not kill them with his own hands, which is why he came up with that whole plot. Sure, he put himself in a huge danger, bit he was so sure of the success he didn't bother to think about consequences.
He left KM's house for barely 5 minutes, just to lock them in, and returned there so fast that not even Bun Tzi noticed he was gone.
While the three are dealing with their fate, he also decides to take profit of the situation to do another thing he wanted to do since a while.

As a KoZ, he has powerful mental powers, but he is unaware of it. He can't use them, there is only one thing he learnt how to do: force people to fall asleep. He does it to Bun Tzi, leaving him on the couch that way, looking as if the boy simply fell asleep there.
Done that, he goes to straight to Persida. He has a few words with her, always acting totally gently and polite in her regards, and when she expects it the least, he uses his powers on her too, to make her fall asleep. With that done, he takes her in his arms and teleports away, taking her to a mysterious place. He leaves her there and, for the moment, returns to the North. He can not risk Sao to find out he left his duty, even for just a couple of minutes.

At home, he sits on a chair in front of the couch, staring at the sleeping boy like a guarding dog.