I think this song is about sacrifice. The speaker believes she could give up her dark side and be all her lover wants "and more and more." But that level of sacrifice is destructive. If you give all of yourself up what do you have left? I think this scares the speaker and she realizes there will always be a siren song (her dark side) calling to her. Towards the end of the song we feel her accepting that she can't be what her lover wants. She exclaims "cause I'm evil..."
I think this song is about sacrifice. The speaker believes she could give up her dark side and be all her lover wants "and more and more." But that level of sacrifice is destructive. If you give all of yourself up what do you have left? I think this scares the speaker and she realizes there will always be a siren song (her dark side) calling to her. Towards the end of the song we feel her accepting that she can't be what her lover wants. She exclaims "cause I'm evil..."