Late to the party, but I really enjoyed Demon 79. I know Black Mirror as an anthology doesn't do follow ups to its episodes, but I wouldn't mind if they made a one-time exception for this one.
What if, after crossing over, Nida learns that Gaap had been lying to her about all of it? He'd placed a wager with Satan over ruling earth...if Gaap could get an otherwise good human to murder three people in as many days, Gaap would take over as earth's new demonic ruler while Satan moved to another world. And with a kicker--if he could then get this human to join him in eternity, Gaap would take over all of Hell for the universe. Obviously Gaap had also lied about Chris not counting for Nida's killing spree to make her choice in joining him easy.
Satan took the bet. Gaap had previously ruled over a 'boring' world filled with good souls...it was basically untouchable by evil, and Gaap couldn't stand it anymore. Had Gaap failed in the bet Satan would have laid waste to both Gaap and his world of innocents, and he found this idea irresistible.
Nida actually triggered the destruction of Earth by fulfilling her kills. Satan had already been close to achieving this himself, he just needed a last push of evil in the form of a demon corrupting a good human to finish the job. Gaap hadn't known Earth would actually be destroyed and was somewhat pissed off when it began to happen. Earth was to be his after all.
Nida learns all of this after crossing over, and she's angry and more determined than ever. The demon power she gains allows her to appeal to the Council of Hell, whom she convinces that the entire bet is invalid since Satan destroyed the Earth. Both Gaap and Satan are banished to oblivion, and Nida takes over Gaap's innocent world as a benevolent demon.
The more I typed this the sillier it sounded, but for some reason I was compelled!