Bruce Wayne's fight with Deathstroke in Ben Affleck's unmade Batman movie challenged him even more than his Batman v Superman: Dawn of Justice conflict with the Man of Steel. Story details about Affleck's Batman movie have painted a fairly clear picture of what Affleck had intended for the film prior to his departure. His conflict with Slade Wilson a.k.a. Deathstroke (Joe Manganiello) in the film is set up at the end of Zack Snyder's Justice League, and it was to have greatly tested him.
As revealed by Manganiello, the story of Affleck's Batman film was similar to that of The Game, with Slade out to wreak havoc on Bruce Wayne's life, Slade having what he describes as a "personal" vendetta with Bruce at the end of the Snyder Cut. Though Batman had previously defeated the Man of Steel (Henry Cavill) in Batman v Superman, Slade's methods and skills would push him to an even more trying brink.
Though Batman and Superman each view the other with distrust for most of the film, Superman was not out to either kill or psychologically torture Batman, and by the time of their actual fight was seeking to reason with him to save his kidnapped mother. Additionally, though Superman had an obviously tremendous power advantage, Batman was able to use Kryptonite gas to weaken him to human-level strength. While Superman still put up a very strong fight, Batman, as a highly seasoned martial artist, was ultimately the more skilled fighter of the two. Batman's showdown with Slade, however, was another story.
Would Batman Have Defeated Deathstroke (& At What Cost)?
Unlike Superman, Slade had every intention of not only murdering Batman, but making him mentally suffer as much as possible first. Deathstroke was also a skilled martial artist himself, wielding a sword - as seen in Jay Oliva's released storyboard for Affleck's Batman film - against the Dark Knight, and as Manganiello has described, overwhelming Bruce with his speed and skill. Batgirl was also intended to come to Batman's aid against Deathstroke, but the chronology of their battle in Affleck's Batman arc also made it a greater challenge than his Superman fight in another way.
Batman v Superman and Zack Snyder's Justice League focus Bruce Wayne's story on his fall into more brutal methods of heroism and his redemption by Superman. With Slade described by Manganiello as destroying Bruce's life and assassinating those closest to him, Affleck's Batman movie was to be a test of not just the Batman's martial arts and fighting skills against Deathstroke, but also his fortitude. Slade's mission against Bruce would push the Dark Knight to his breaking point, as even in victory, Affleck's film would test whether Batman would fall back into the abyss he had been in before being saved by Superman.
Snyder's Knightmare future, seen at the end of Zack Snyder's Justice League and forming the foundation for his Justice League sequels, would see Deathstroke as one of Batman's allies in undoing the apocalyptic timeline. With Affleck's Batman movie set between Zack Snyder's Justice League and Justice League 2, this clearly indicates that Bruce did not succumb to the urge to kill Slade. With that said, that temptation was going to present itself to him in Ben Affleck's solo Batman movie, with Deathstroke's challenge as a villain surpassing the conflict of heroes that the Dark Knight and the Man of Steel had at first met one another on.
