[... It's fine. It doesn't matter here, anyway, so she'll be honest.]
In order to turn another human requires a sacrifice. In order to achieve it, you have to forfeit your own life.
[He gets it now, right?]
If Kaname annihilated the pureblood race, and then turned me human, it would kill himself. There'd be no more purebloods left, and he wouldn't have to physically harm me.