The Tyrant's Substitute Bride, the Little Poisoner

Chu Luo was a genius poison master in the 21st century, but one day she transmigrated into a poor girl whose wicked elder sister had disfigured her and forced her to take her place in a marriage. Rumor had it the crown prince was brutal and merciless, and everyone gleefully speculated about how many days his new crown princess would survive. But no one expected Chu Luo not only to survive, but also to heal her face and win the heart of the peerlessly elegant man. A succession of jealous, wicked cannon fodder characters kept showing up, only for the crown prince to smile faintly and run them through the chest. Then he turned to his darling wife, hiding the bloodstained handkerchief and sword behind his back, and smiled with tender innocence.

“Didn’t you promise to stop killing people so casually?”

“Be good. I was afraid your hand would hurt.”