Demon 79 is perhaps the best Black Mirror story of all time, and also probably the most misunderstood.
A lot of people were put-off with the ambiguous nature of the story. Was Nida really crazy or was Gaap the Demon-intern making her do the murders? Did the nuclear attack really happen in the end, or was it just a figment of Nida's imagination?
These questions are great, however they seem to miss the fact that the ambiguity IS the main point of the story. This episode brilliantly uses it to drive home the main message of this story.
To establish the ambiguity it gives us evidence of Nida's descent into madness - her loneliness, the racism around her, a possible brain disease she genetically inherited from her mother - all are presented as possible causes.
However the story also provides evidences, sometimes subtle and sometimes not, about the supernatural element as well. Nida is an Islamic immigrant living in England in 1979. Now ancient Persia (now Iran) had of the myth of the Djinn, an entity which famously grants three wishes when the user encounters it's associated talisman (wishes would be with or without loopholes depending on the iteration of the story). It is therefore not far fetched to think that Nida inherited a generationally passed-down talisman from her mother, used it to summon Gaap - an evil alternate version of the Djinn, who demands three kills to prevent eternal doom, instead of granting three wishes to achieve eternal glory. We outright see how Gaap helps her choose her targets, and keeps her on her path to 'save humanity' by giving her accurate butterfly-effect consequences of her present decisions.
This established ambiguity is critical to drive home the point of the story, that it does not matter if she's crazy, or there's a supernatural entity - a devil on her shoulder - making her do the murders. It does not matter if a nuclear attack really happened or not. What matters is how we - the audience - react when we see what she does. We applaud her when she delivers justice to the paedophile and the wife murderer. We support her in her endeavour to kill the racist-in-sheep-skin politician. We even take guilty pleasure in her killing the brother of the wife-murderer - who symbolically represents the apathetic society that turns a blind eye to criminals among its own. And when she fails to kill the last person, the World itself is burned, as if to symbolise that a World without justice is a world that should not exist. The story is ultimately a reflection on how the human consciousness craves for freedom, justice and equality. Does it really matter whether it is delivered by a crazy person, or by an agent of Divine Retribution?
That's why it's perhaps the best Black Mirror story of all time. No other story holds up the 'Mirror' to society as well as this one.