But that would basically be killing the golden goose before it can even hatch you a few eggs. She knows a lot and obviously that's a security risk, but at the same time it can be a potential boon. If he manages to get on her good side or she manages to prove her worth, then keeping her alive could have far more benefits than killing her.
Plus, it's not like she's demanding all that much. Just a few months of playing fiance and he can gain access to all sorts of forbidden information. If anything he's benefiting more than she is.
Of course, she could be playing on what she knows of his character. From the looks of it, the ML seems like the type to "play with his food" so to speak. Perhaps she's playing on his innate sadism so as he is more willing to play along?
hy would he not kill her if she knows that much? he looks like the kind that will get all the info he can from her then VANISH her for good. Just kill her when she can't give him more