I've said it before in other threads, & it makes less than 2 fornications of difference to anyone, but I think I saw & really appreciate what Snyder was trying to do...
Superman's film was another origin story, but instead of him being the boy scout because of his upbringing, it was almost in spite of it. The Kent's treated his powers as something to be ashamed of. They were so selfish & misguided, than instead of recognizing the good that their son could do, they wanted it hidden so... What?... He might get taken away?
Who was going to be able to do that?
He might turn out bad?
I mean, they felt so strongly, PA was willing to die to prove he was right.
Clark became who he was because that's who he chose to be. Now about the killing, I think that he also had to have that moment where he decided to become the boy scout we know as the character. In his first outing, he felt he had no other choice than to kill Zod, but as David Goyer recently said, that was to be his one & only kill. After that he's determined to find other means to resolve the issues he faces, so in MoS, we see the first steps of him becoming the Superman we know.
Now as far as Batman goes, it's the opposite... He's been through so much personal loss, probably after dealing out mercy time & time again, he has the attitude of, eliminate the problem, they'll be no repeat problem. In that, he's basically militarized his MO to the point that it's the mission that matters & collateral damage is just a byproduct.
He needed that kryptonite. These guys have chosen a life of serving Luthor, so they've chosen to put themselves in the situation they're in, so if in accomplishing his mission in the most expedient manner, they die, so be it
How many times do we think he sent Joker to Arkham before he killed Robin? He's just broken.
Then we get to the point of BvS, IMO.
To Superman, he had to be willing to die for the people that his parents had repeatedly told him didn't deserve him. He had to be willing to die for the people that he was taught would be afraid of him & try to destroy him. Even when confronted face to face with Bruce, seemingly proving his parents point, he was still willing to not kill, so that lesson was learned, & was still willing to die to save them all.
Seeing this, is what brings Bruce back. It inspires him to return to his former ethical & moral code. This is shown at the end when he doesn't brand Luthor. It was said earlier that the branding was basically a death sentence in prison. He didn't physically kill the person, but the brand would ensure that they'd still die, thus eliminating the problem. Seeing Luthor willing to let Innocents die for the 'greater good' also snaps him out of it, which would bring them into JL.
I'll stop here, but this is where I felt that Snyder was going. He was bringing us to the Justice League we all wanted to see, but each character was going to be earning their place as a hero. It wasn't just assumed because of decades of history that we all know. I try to look at a film from the standpoint of the what the characters know, not what I do.