One of the most highly anticipated comic book movies, Batman vs Superman, wasn't even supposed to be about the two men fighting against each other. The movie is a sequel to Man of Steel, and originally it was supposed to be even more so, without Batman even making an appearance.
Director Zach Snyder actually just brought it up in passing during a meeting.
"'I remember we were talking about 'What's the next villain'" Snyder says. "'We can't do another alien invasion.' Brainiac was definitely down the road. Metallo, I think, was going to be the main bad guy of this movie. I said, 'What about at the end of the movie we do a scene where there's a crate full of kryptonite delivered to Wayne Manor.' Everyone was like... 'Okaay.' Once you say it out loud it's a problem because you can't unsay it."
I guess that the writers and people behind the movie were just worried about more destruction to Metropolis is there was another alien invasion and Superman was left to his own devices.
Batman vs Superman will be battling into theaters March16, 2016