I've watched the longer cut of BvS a few times now, and I think the movie the filmmakers were trying to create went over the heads of most movie-goers, especially since it can be argued that their ambition exceeded what they were able to pull off from a credible story-telling point of view.
The arc of the two characters is built around how they contrast one another. Superman is almost physically impervious to harm but emotionally sensitive, evident by how deeply he cares for the people in his life, which only burdens him as his exploits become more public and the world begins to look upon him as a savior. Batman, on the other hand, suffers a great emotional loss as a child, works towards making himself physically powerful while distancing himself from others, until his lifelong war on crime begins to corrupt him ("We're criminals, Alfred. We've always been criminals."), with few friends and little hope towards the future.
So, when our heroes are introduced to one another, they only catch a rather uncharacteristic glimpse of the other. Superman, in his blind rage battling Zod, brings wanton destruction with great collateral damage, leaving Batman with understandable concern that this may only be a sign of things to come. And Batman, with "a new kind of mean in him," is seen by Superman as someone who is allowed to pursue his enemies as he sees fit, unopposed by the police and the justice system.
Thus, when Superman sacrifices himself, Bruce Wayne begins to recognize his sins, decides to form alliances, and re-examines his brutal methods. In the course of the two films, we see Superman saving humanity in many instances from mortal danger, but here we see him saving Batman's soul. He has to be brought to the darkest depths so Superman can bring him into the light.
If the DC cinematic universe is to play the long game, it shouldn't begin by showing our heroes as audiences have come to expect them portrayed right out of the gate (that's just fan service, and there's plenty of that going around already!), but where they are as three-dimensional characters first. That way, it leaves us with more than just learning how they get their powers and why they wear the costumes they do.