[ She nods, but: ]

And yet I cannot let you die. I will not. [ There's a difference, even if the result is the same. ] It is my duty as your Servant to value your life above all else. Just as it is your right as my Master to stop me.

[ It's implicit in her words: that Irisviel will have to use a command seal, if things come to that. She'll give her as much time as she can; but in the end, she will have to act. ]