Lady Onion was after all an onion spirit that had lived for 300 years. She had accumulated a huge amount of monster qi within her body—even though all that monster qi was of no use to her, and could only be used to scare human beings.
However, monster qi was still monster qi!
After someone other than a specialized immortal chef chopped her up to use in cooking, the monster qi wouldn't be removed due to lacking skills of the cook and enter the body of the person that consumed the food.
For ordinary human beings, they would be infected by the monster qi after eating the contaminated food.
The lucky ones would become deranged fools and then start killing people, stirring trouble, etc., until they tormented themselves to death.
The unlucky ones might end up becoming half-human and half-monster after being infected with the monster qi… by then, they would eventually die from unknown causes.