The Snyder Cut of Justice League was indeed quite a different film, including a scene that makes Flash responsible for the Knightmare future. Zack Snyder's 2016 movie Batman V Superman: Dawn of Justice left fans with many questions in need of answers, and not all of them were related to why Jesse Eisenberg decided to play Lex Luthor so strangely. The biggest question marks revolved around the Knightmare sequence, in which Bruce Wayne experiences a vision of a coming dystopian future, followed by a time-traveling warning from The Flash.
In a parallel to then-recent video game Injustice: Gods Among Us, Batman saw a future in which Darkseid had taken over the Earth, and had used the Anti-Life Equation to dominate Superman's will and turn him evil. That's what audiences know now, that is, after watching the Snyder Cut of Justice League. The theatrical cut of Justice League cleared up absolutely nothing about the Knightmare timeline, as, after Zack Snyder's departure from the DCEU, his plans to continue the storyline were abandoned and the studio demanded significant changes be made to the story by the director who took over, Joss Whedon.
While The Flash told Bruce in Batman V Superman that Lois Lane was the key to why things turned out the way they did in the future, it seems he's remembering things a bit too vaguely. While Lois's death may have made Superman vulnerable to Anti-Life, a simple change in course by The Flash in the Snyder Cut could possibly have averted the Knightmare completely.
Justice League: Flash's Mistake Led to The Knightmare Future
In the Snyder Cut of Justice League, Cyborg experiences his own horrific vision of the Knightmare future, right before the team of DC heroes awakens a Mother Box in order to resurrect Superman. Stunned, Cyborg then softly says "no," but The Flash, standing by with orders to build up enough speed to jolt the Mother Box awake, mishears the word as "go," leading to Batman's plan moving forward and Kal-El being brought back to life. While Superman does end up playing a decisive role in the battle against Steppenwolf, it doesn't change the fact that The Flash may have made a colossal mistake.
The Kryptonian ship repeatedly implores Cyborg not to go through with awakening the Mother Box, arguing that doing so would be quite ill-advised. Batman argues against the ship, but based on Cyborg's reaction to his vision, he probably would've reconsidered their plan. The Flash's mistake takes this option away, sending them down the path toward destruction, a path it might not be possible to undo. While it's unclear if the Justice League could've beaten Steppenwolf without Superman's help, it's certainly not impossible that they could've devised an alternate strategy that didn't involve the Man of Steel. Instead, Superman is set up to be both their savior and undoing.
