Lilith wasn't the one killing the OCS sisters. The FBC was.

Lilith was on a mission to find Vincent and Mary. She was checking in regularly with the OCS until they were attacked and had to go underground. At that point there was no way for them to tell her where they were. Meanwhile she's like wtf is happening to me, I must consult science. When she goes to the other realm, all she remembers is being scorched in an endless desert for hours, and then when she gets back covered in burns Jillian immediately wants to send her back. She refuses and says she's going to get answers elsewhere. She turns to Adriel because Vincent tells her that he can give her answers. He plays on her resentment for Ava getting the halo