@Nolwenn I judge this to be quite a realistic depiction of what sort of personality a person would develop if she was encouraged from a small child to abuse her own little sister in preparation for some wicked ritual. Basically it means she was brought up to believe nothing has any value beyond utilitarian, that is, one can even sacrifice their own family for their own benefit. Of course on a more basic level it would have also made her cold, manipulative, ruthless, and traitorous, seeing how lies are a second nature to the Sperados.
I wonder if there was some solid reason why the Sperados needed to allow Leslie to grow up in the same household. Couldn't they have paid some ruffian to handle it in some forest cabin? That way Leslie would have learned absolutely nothing she wasn't supposed to learn and couldn't have run to the Salvatore estate either, knowing nothing of the world or her own background. Who knows, maybe the transfer of power ritual required some closeness, even if it was rotten closeness.