One dozen possible reasons/scenarios for why Madam Lan couldâve killed Qingheng-junâs teacher that do not, in fact, involve self-defense from sexual assault.
They happened to work together on a night-hunt, but at the end of it, the teacher tried to discredit her to get all the glory for themself or they made some sort of mistake and tried to shift the blame onto her, and she then killed them.
They happened to work together on a night-hunt, but the teacher got possessed by a wrathful spirit that tried to kill her, so she ended up killing the teacher in self-defense.
In a similar vein, there was a night-hunt that involved illusions and hallucinations that made people confuse friend and foe, and she killed the teacher by accident, thinking they were an enemy.
She couldâve had a morally dubious relative, perhaps one of her parents, who was doing harm to others, and the teacher killed them. Then she killed the teacher in revenge.
Again, in a similar vein, her loved one died and came back as a resentful ghost, and the teacher destroyed their spirit before it could harm anyone, but barely even trying to see if their spirit could be liberated. Then she killed the teacher in revenge.
There was an incident where a group of people were doing crimes, and the teacher wanted to punish everyone involved, even if they were blackmailed into doing it, or were children of the ones complicit, and she killed the teacher to protect them.
The Lan Clan laid claim to a spiritual artifact that was dangerous, whether recently or generations ago, but it was a family heirloom to her, and there was a lethal altercation when she tried to reclaim it.
She had a relative or friend whose life was in danger, whether by illness or curse or whatever, and the Lan Clan had an artifact that could save their life. So she went to go steal it, leading to the lethal altercation.
The teacher prioritized someone who was more affluent, but less threatened, over the immediate danger of someone elseâs life (ie: standing as bodyguard for a rich patron, who is already has some minor protection from talismans and arrays, while someone outside of those protections is actively getting chewed up by a yao, or similarly, two people were cursed, but the teacher prioritized lifting the curse from the rich patron, who was afflicted only recently, over someone of lower rank, whose curse had already progressed much farther), resulting in the death of a loved one of the Not-Yet-Madam Lan. She then killed them in revenge.
A similar incident to what happened with Lan Qiren, Cangse Sanren, Wei Changze, and Jiang Fengmian couldâve occurred, where the teacher was so caught up in making sure that things went according to procedure that they ended up endangering the lives of others. Except this time tempers flared, leading to a lethal altercation.
Taking a page out of TGCF, the Not-Yet-Madam Lan, or maybe even both of them, were cursed, and the only way to break the curse was through bloodshed. The teacher may have offered to sacrifice themself, or maybe she just killed them to save herself.
Either the Not-Yet-Madam Lan or a loved one of hers was cursed or simply afflicted with a mundane malady. So she sought help through a dubious source, like a resentful yao or demon or a malevolent god (a la the dancing stone Heavenly Maiden) to have her wish granted. The teacher learned of this on a night hunt, and they came to cross blades, either because they tried to destroy the dubious âhelperâ (for lack of a better word) before her wish could be granted, or they attacked her directly for fraternizing with evil beings.
And a bonus scenario to bring it up to a nice bakerâs dozen rather than a regular olâ dozen: Sparring match gone wrong.
It couldâve been as simple as getting a little too overzealous in a sparring match that ended in a fatal accident.
Iâm not saying that the common headcanon isnât a possibility, but itâs just that. A possibility. Thereâs a myriad of different reasons for why someone would kill someone else, and we donât know anywhere near enough about the circumstances to say âThis is definitely, for sure, what happened.â