And one of the more unpleasant enclavers from Beijing brushes his hand against the wall of the stairwell and is immediately devoured by a clear transparent gooey substance. 

Mingjue is quick with a spell but not quick enough-- he'll get it before it hurts anyone else but not before there's no chance of the victim surviving--

It is so satisfying to see the expressions on the enclaver's face through the transparent mal. 

And Nie Mingjue's eyes flick up and he sees the smile--

And he knows, suddenly, knows bone-deep what Meng Yao has done-- he fires off the spell without even thinking, kills the mal--

"Why did you kill him?"

Confusion or obeisance--

Meng Yao hits the floor.

"This humble servant begs his senior's mercy for this unaccountable error."

The other people in the classroom are all too stunned by the death to have any sort of reaction to what happened.

"No, it's not-- mistakes happen, you killed him."

"This humble servant's carelessness is--"

"Not your carelessness, I saw your smile-- you wanted him to die."

"This humble servant doesn't want the death of any ally of Shanghai."

Nie Mingjue knows what he saw. The slimy, disingenuous, lying little dog--

Meng Yao is right by his feet. Nie Mingjue kicks him down the stairs. 

He tumbles-- he can feel the sharpness of the steps against his skin-- there'll be scrapes at least, maybe bruises-- he's good at his job, he's very very good at his job, if he weren't he would be dead right now, he's glad he wasn't ambitious enough to aim for two murders--

It is good to note those things, because otherwise he would feel something about Nie Mingjue, the second person who was ever kind to him, kicking him down the stairs, and he cannot afford to have feelings right now. 

When he reaches the bottom, he arranges himself again in a kowtow.

"I'm going to get Xichen."

"Please allow this humble servant to escort you in case there's trouble--"

"I don't want any help from you ever again, you son of a whore."

He races off to find Xichen, studying in the library.

Breathless, he says, "Meng Yao killed someone."

"Oh?" Xichen says calmly.

Xichen is pathologically willing to believe that there are good explanations for things done by that treacherous son of a whore.

"There was a wall jelly on the stairwell. Meng Yao claimed the stairs were clear-- it ate a Beijing enclaver--" 

"I am sorry for their loss. But wall jellies can be hard to spot-- accidents happen, da-ge--"

"You don't understand, you didn't see the way he smiled, like he wanted it to happen--"

"And then what?"

"Huh?"

"...do you have any evidence for this accusation other than a facial expression?"