Warning: The following contains SPOILERS for The Flash: Armageddon.
feature-length animated Injustice movie.
The main story of The Flash: Armageddon centers around an alien named Despero, who traveled back in time from the year 2031. Despero attacked The Flash, claiming to have witnessed a great evil destroy the Earth and that this evil was a Flash gone mad. Despero also claimed that Barry Allen's madness came about because of one bad day in which his life fell apart. The Flash season 8 episode 2, "Armageddon, Part 2" detailed this day, which ended with STAR Labs being shut down for environmental safety violations and Barry Allen facing federal criminal charges. The episode ended with Barry meeting Black Lightning at the Hall of Justice and, when asked what was wrong, replying with a single word, "Injustice."
The base idea of one bad day turning a virtuous hero into a villain was most famously explored in Injustice, where The Joker set into motion a chain of events that brought about the death of Lois Lane and the destruction of Metropolis. Worse still, the Clown Prince of Crime set it up so that Superman was responsible for the death of his city, his wife and their unborn child. This led to Superman becoming a tyrant, determined to prevent anyone from suffering as he had, by any means necessary, even if it meant policing the world himself with an iron fist. While The Flash season 8 Armageddon event seems to have taken a similar turn, there are still some sizeable differences between it and the original Flash Injustice storyline.
The Flash Replaces Superman In The Arrowverse Injustice
The most obvious difference between Injustice and Armageddon is its main characters. Injustice was centered upon Superman, who is most often seen as the most noble and virtuous of heroes and the one most heroes look to as a leader. Barry Allen holds a similar role in the Arrowverse, so it makes sense for him to be the focal point of an Arrowverse adaptation of Injustice.
Some might wonder why an Injustice adaptation might not be saved as a storyline for adaptation of Injustice, with an alternate universe version of Superman killing his world's Lois Lane after betraying the people of Earth. Repeating the same story so closely in Armageddon would be redundant.
The Arrowverse Injustice Isn't Started by The Joker's Death
Another major change is that The Flash season 8 Armageddon storyline was not kicked off by the death of the Joker. In the world of Injustice, the inciting incident leading to Superman's slow slide down the slippery slope of idealism towards villainy was his cold-blooded killing of the Joker, as the Clown Prince of Crime laughed his last laugh. The Joker does not figure into the story of Armageddon at all and indeed could not, as the Arrowverse version of Joker was reportedly killed off by Batman at some point in the past history of Earth-Prime.
The Cast of Characters is Wildly Different
Another major difference between Injustice and The Flash season 8 Armageddon event is the cast of characters. The comics tying into the Injustice games explored the effects of Superman's choices upon the whole DC Comics universe. Over the course of five years, writer Tom Taylor explored how the New Gods, Earth's magicians, the ancient gods and intergalactic peacekeepers like the Green Lantern Corps all made a stand against Superman's increasingly authoritarian behavior. Yet the comics also told a number of deeply personal stories, with the redemption of Harley Quinn and the sacrifice of Oliver Queen being among the most notable.
By contrast, the Arrow season 2 and Oliver Queen having already sacrificed himself to save the Arrowverse during Crisis on Infinite Earths.
There Probably Won't Be Any Alternate Earth Doppelgangers
In the world of Injustice, Superman's regime was finally brought down after the Batman of his world made with the Justice League of another Earth, presumed to be the main DC Comics Earth-0. The first game, Injustice: Gods Among Us, detailed how this Justice League fought to free the world of Injustice. They were aided in their mission by that world's versions of Batman, Superman's archenemy Lex Luthor (who had served as Batman's mole among Superman's allies) and Barry Allen, who had come to regret his actions aiding Superman in building what he thought would be a better world.
It seems unlikely that a similar group of superheroes from another Earth in the Arrowverse will show up to resolve the story of The Flash season 8 Armageddon event. The main reason for this is because the current rules of reality following Crisis on Infinite Earths make traveling between dimensions much more difficult and Earth-Prime seems actively hostile to doppelgangers from another Earth unless their Earth-Prime counterpart is already dead. This was shown in another Arrowverse superhero team from a different world, like in Injustice, rather than The Flash and his allies.
