Wei Ying signed up for Monday morning shop and it is very upsetting. Wen Qing and Jiang Yanli seem to have it handled but Lan Wangji is very upset that Wei Wuxian didn't consider the most important aspect here, which is that he's the center of Lan Wangji's life and without him the world would be cold and empty and without joy. Wei Wuxian isn't aware of this-- Lan Wangji has in fact tried very hard to keep him unaware of this, because it would be embarrassing for everyone involved-- but Lan Wangji is still inexplicably angry about it.
Lan Wangji wants to stab something.
Specifically, he wants to stab the mals on the shop level, because every single one of them has personally offended him by counterfactually killing Wei Wuxian.
He is aware of his hypocrisy here as regards the taking of unnecessary risks but Lan Wangji is hardly the center of and only source of joy in his own life, now is he. His death is primarily bad insofar as it leaves the Keep Wei Wuxian Alive project in the hands of Jiang Yanli and Jiang Wanyin. Besides, Bichen has strong opinions as regards being used to take the lives of evildoers and while there is a human evildoer running around Lan Wangji has no idea who it is, so he has to kill mals, and he might as well kill the ones that are personally offensive.
So he spends the after-lunch work period on the shop level looking for mals.