Zack Snyder planned to kill Batman at some point in his original five-part DC Extended Universe series. The truth was revealed when Ramesh De Silva posted an image of Superman holding the lifeless body of the Caped Crusader, which was inspired by Final Crisis, the crossover storyline comic series for DC Comics written by Grant Morrison. De Silva suggested that Batman’s death could have been "what we may have seen in [Zack Snyder's] 5 story arc."
Snyder didn’t appear to mince words either, simply responding, "Of course."
Justice League concept artist Jay Oliva alluded to the idea that Snyder already had a plot in place for quite some time for a five-part DCEU series, starting with Man of Steel.
ScreenRant has pieced together what they believe to be Snyder's original plan from Justice League and how it would lead into its sequel. The inclusion of the evil ruler of Apokolips, Darkseid, in Justice League would serve as a set-up for the next film, which is presumably where Batman would sacrifice himself at the hands of Darkseid.