He could also ask her to get close and cut her tongue out, but nooooo
Or agree with her in the front, and secretly poison her food after...
This!
I agree with both Ayaben and Sakuraruu. She obviously is going to use that to continue to blackmail him and make more trouble and chaos. He could just make sure she can't say anything when executed like the tongue or destroy the vocal cords in some way or heck just gag her with cloths to "spare" the citizens of her nonsense. Also when she had gone to fmc I was thinking why didn't they capture and torture her or when not when she entered when she left to not get suspicious with her disappearance.
From the flashbacks it seems he was more sacred of her more than his life of slavery. Like she’s so crazy, his fear seems to be som sort of conditioned response/trauma
It doesn’t make sense that he would allow her to go free to be able to blackmail him. If she was going to get executed the next day, all he had to say if she tried to reveal their relationship is that she’s a liar who tried to sell someone else’s organs for jewelry. They could’ve just went with something like him feeling guilty that he left her because this one doesn’t really make sense given that she was going to be dead literally the next day.