Theory: I think Fiona’s role may be bigger than we think

What if Mr. Nas releases Fiona from Hell on Devil’s Night, expecting her to help the serial killers against Robichaux’s?
He underestimates the one bond Fiona cannot sever: her love for Cordelia. Fiona may be ruthless enough for Satan to see her as an ally, but when the killers attack, she turns on them to protect her daughter and the Coven.
And if the theory that Fiona has some kind of past history with Mr. Nas/the Devil is true, that could give her an even more personal reason to undermine him. Maybe she already knows exactly what he is capable of, or has unfinished business with him.
That could also explain “The Baddest Witch of Them All.” Mr. Nas thinks he has unleashed one of Hell’s most dangerous witches as an ally, only to discover that she’s the one who ruins his plan.