Here's the deal. Sylas is, in my opinion, every nightmare, every feeling of hatred, despair, depression, dislike, distrust, sorrow, regret, and fear that exists in the Core of Lux's character--that is, her dislike for what she is, her hatred for her lack of full control, and her fear that one day her power will escape her control and actually hurt someone. Using her power, Sylas hurt people. He killed people. With her power.
As someone who didn't have to resort to violence to make herself known, I don't believe that Lux has any positive feelings for him. I don't even believe that she has any negative feelings for him. Not only did he betray her, but he caused her to betray herself in the worst way. Yes, she led to his escape and him hurting people, but he also led to her powers hurting people.
He broke all the effort she ever put into bettering herself for her people. If she sees him, pretty sure she's going to completely obliterate him. To obsolve herself. To save her people. And to save him from himself.