I disagree. I think the first 20 minutes was all we needed to see why Bruce Wayne goes to battle with Superman. What we missed was an establishment of Batman's position in the world. One scene establishes him as a new vigilante, unknown to the policeforce, the next makes us believe he's been around for a while, another makes you wonder if he's retired... there's never a sense of Batman's presence in the world.
For instance; the sex-traffickers. The cops show up and they find the batarang in the wall. It would've been perfect if they'd relax and say "ah, good, the bat is here." or "what the hell is that?" or "is he back?". It establishes Batman's connection to the world.
Instead, they draw a blank and fire on Batman when they see him. Now that COULD establish something, but it also allows for an interpretation that he fired in panic.
For instance, take this scene from The Dark Knight Rises:
https://www.youtube.com/watch?v=EBicn4PqLFY
it establishes that Batman is back, how the cops feel about it, and that he's a known entity to the policeforce, without relying on the previous 2 films. All that in mere seconds.
Similarly, you could establish his no-kill policy and how he drifted from that (although they do allude to that in the scene with "20 years, how many were good, how many stayed that way?", it indicates that he's fed up with his lack of real accomplishments).
Instead, after the first 20 minutes, the movie assumes you know Batman, not only that, you know Snyder's interpretation of Batman, and that's where things go sideways.
