Batman in the movie is struggling with his own insignificance. The notion that nothing he does will ever change things and the only thing he really managed in these last 20 years was to ruin the lives of those around him, specially those that believe in him. That the dream that he could ever be change things for the better is a lie. Him being a hero is a lie. His crusade has and always was a meaningless effort. There are no heroes. Those that believe so, like him, are only being naive and blind to their own hubris. If there are heroes, it's only people like his father. Normal people bleeding on a sidewalk trying to protect their own family.
That's why Batman started to brand people. He wanted to make his efforts matter and last. So he started to brand criminals so their sin would forever be printed on their skin. Then even that wasn't good enough and he started to outright kill. They can't commit any crimes if they're dead. Batman doesn't care anymore. He never was a hero to begin with and his aspirations were nothing but lies. Let them die. But then nothing even killing criminals was enough. Criminals are nothing but weeds, you pull one out, two more spring up.
So what can Batman do that will truly matter? That will legitimize all these years of empty struggle? Well, there's Superman. Someone as blind as he was who's fucking the world in a monumental scale stupidly believing his going any good. What will happen on the day Superman realize the futility of his actions just as he did? Will Superman go on a fruitless killing-spree like him?
So if there was ever something Batman could do to make his life and struggles matter for anything, it is to rid the world of Superman before it is too late. And hey, maybe he can die doing it too. Wouldn't that be great. Two birds with one stone.
And that's essentially Batman character in the whole movie, which i found to be an interesting take. What happens when Batman loses hope in his crusade?