Lily should be happier. Not only did she more than get away with it, she also got a healthy heaping of everything she could have possibly wanted to result from it beyond her stepfather’s corpse. To say that she was unhappy would be incorrect. It was just that she wasn’t entirely satisfied.
Ever since that night she killed him (and Amanda drank every drop of the lemonade, why would she-) she craved more blood, more carnage, more death. This really took her off guard. She wasn’t expecting that. Lily assumed that it was just about Mark, but now she was filled with a bloodlust and no one to aim it at.
To call it frustrating would be oversimplifying the matter.
Lily would have also wanted Amanda if she didn’t already have her. Maybe it wasn’t in a way that she would have previously imagined or that anybody else could understand but it was undeniable. Lily had Amanda in a way no other person has had someone else before.
(She would never say it, but having her by her side would have also been nice. But unlike murder, as there were more than enough disposable people in the world, that would be something Lily could never have again if she knew what was good for her.)