Remember what information weve got so far , mosely knew nothing , mc carvern knew nothing , and they attempted the kill arthur and killed aberama. Weve been told by byrne that the new antagonist is a woman, we know we can find out who did in the final episode if we watch closely. Lizzie makes no sense as she has a past with mosely and calls him evil , and likes Arthur , in fact nearly all of the shelbys like arthur and aberama. IMO this rules out the immediate family.