Two desires filled Lucius's being at that moment, the first was to pull her close and hug her. To protect her because she was his and had always been his but protecting her right now meant making sure she didn't find out what he had done. So he had to push her away.
The second desire was far darker and caused a glint in his eyes in response to what she was saying. He hadn't considered it previously but she was right, Slayer's blood, her blood would be extremely potent. The power he could obtain from just a small cut, his pulse quickened just to think of it.
"Is that what you want? For me to hurt you?" Lucius asked as he stepped towards her, very much aware of the presence of his knife sheathed in his pocket. Doing it would probably mean more interfering from Buffy on Evie's part, but the power... the sweet darkness...