Labels are an interesting thing, aren’t they? They can help define us, but they can also limit us. Did Lucifer give himself that label of “monster”? Did his Father make him into a monster? Lucifer doesn’t seem to think so.
During the very last episode of season 3, we see Lucifer define himself in Cain’s presence. Cain didn’t deserve such a revelation, nevertheless, Lucifer told him, almost as if knowing, that he would succeed in fulfilling his promise, of ultimately finding a way of killing Cain.
Lucifer sent him straight to hell. But in doing so, Lucifer was so filled with rage, that his devil face was seen by Chloe. As Lucifer turned around, Chloe saw him. She knew. She finally knew that all the stories that Lucifer had told her, were all true. She knew.