Master Gracey and Elizabeth's relationship was "unacceptable" for another reason
Elizabeth was the daughter of a servant in Gracey Manor. An important fact to take note of here is that she is of mixed race, not just black. What if she was secretly the illegitimate daughter of George Gracey, Master Gracey's father?- Then why couldn't Ramsley just tell Edward that? Seems a big step to take resorting to murder, when it's likely a secret like that would be kept.
The flaming snake thing was the actual Devil
In the Book of Revelation, the Devil is referred to as a serpent and he's generally believed to have been the snake in the Garden of Eden story. Lucifer was mad at Ramsley for avoiding going to Hell for so long so he decided to personally come to collect.