I want to look after you
There were only very few instances in Ming Yue's life where she lost her cool and acts out of her character. Given her position in the clan, everyone made sure not to piss her off, and even her family avoided getting into her bad side. It was a well-known fact among the clan members that only those people who were tired of living would get on to the witch's bad side.
It wouldn't be a lie to say that she always maintained an upper hand over her opponents, especially over men. Every time she meets a strong enemy, she looks for their weaknesses and uses them as a weapon to destroy them. Or even worse, she let them annihilate themselves.
She planned the same for Wan Yi Jun, too.
Ming Yue searched for his weakness so that she could use it as a tool against him and escape in case of an emergency.