If the Darkness hates the vampires and is vengeful against them, as Esther had told us, then can't we assume that Qetsiyah is the Darkness? Qetsiyah didn't come when Bonnie summoned her, because the Earth can't handle the Darkness, and since Qetsiyah helped create/channel the energy for the Immortality (Vampire) Spell, but her plan to stay with Silas for ever backfired, she is vengeful towards Silas and that's why she made the Other Side, etc. and she became known as the Darkness.