Robin has been murdered.
Every person he ever trusted (except Alfred and probably Gordon) has turned bad.
Every time he puts a criminal away a new one crops up.
He is middle aged and past his peak.
I think every bit of evidence goes to show that this is very much a direction Batman could go. Batman is not a moral person. Batman is a criminal that convinces himself that he isn't. But he is. He doesn't strive to help people, he punishes criminal first and foremost.
You're absolutely right. Realistically, this is a direction that Batman
could go in, but shouldn't go in, and that was the idea. Especially in regard to the notion I bolded, which is something Bruce actually says in the film. I imagine if we met Affleck's Batman at an earlier, more hopeful point in career, he wouldn't speak or think that way, nor would be refer to himself as a criminal or liken himself to a hunter.
Batman absolutely does want to help people and save people. He believes in the good of humanity, but at this point in his life, he's lost sight of that and lost that kind of idealism. Superman's arrival and what happens in Metropolis is like the exclamation point of that mindset, as he bears witness to an incident that strikes him with the same feelings of powerlessness and rage that he felt when he was a boy and his parents are gunned down in front of him.
Let's not forget that after his parents are murdered, it's not like Bruce automatically became Batman one moment later. There were years -- more than a decade -- between that moment and the beginning of his career as Batman. I think that it's commonly accepted that Bruce struggled for many of those years with his anger and feelings of inner turmoil, and that he was a "lost soul" for some time before training and embarking on his Batman crusade. Even Batman Begins shows Bruce planning to shoot and kill Joe Chill as a teen or college student, before someone else pre-empts him. But eventually, Bruce is able to fuel that rage into something good. He's able to honor his parents by devoting his life, his mind, and his body to saving people, preventing children from becoming orphans like him, making his city a better place, etc.
But here, he bears witness to another act of seemingly senseless violence on a grand scale in Metropolis that he was powerless to stop -- buildings coming down, thousands of people dying including people he knew, a girl who becomes on orphan in front of his eyes -- it's like Crime Alley Version 2.0, but this time, he's already Batman, and a veteran, jaded Batman at that. While becoming Batman had helped him rise out of his own darkness once before (hence the bats carrying him up to the light in his dream), that can't happen again. He's been Batman for 20 years and is past his peak to an extent, already having lost some of that hopeful idealism he probably once had after all he'd seen and experience, and this only makes things worse. He doesn't have another decade to find himself after his world is rocked to its core again in a bad way.
The things that make Batman so great aren't just the 'Batman' aspects -- the suit, the gadgets, the fighting, the skills, the fear-inducing persona, etc. It's the Bruce Wayne aspects -- his compassion, idealism, logic, reason, infallibility, sense of purpose, faith in his mission and the good in people, etc. Take away some of those aspects in a Bruce who is not of sound mind, and you're left with the kind of Batman we see in the first half of BvS --- aka, not the Batman we know and love or ever want to see.
So this time, following the Metropolis incident, the "Bat" persona all but consumes him, instead of saving him and giving him purpose as it once did. He tells Alfred he needs the Batsuit to do recon/detective work, and Alfred tells him he doesn't and must go as Bruce Wayne to do it. Before doing so, he intensely stares at the Batman suit, probably wishing he could use it because he feels uncomfortable doing anything like this without it. He dreams about bringing flowers to his parent's grave site, but the "Bat" creature breaks through his mother's grave stone, refusing to let him honor them or think back to the real reason he became Batman. His methods as Batman become more cruel and callous. He devotes himself to seeking the means to take down Superman -- the being at the center of the tragic Metropolis incident -- rather than devoting himself to seeking the means to fight injustice/crime as he originally set out to do. He doesn't listen to Alfred's logic and reasoning, as Bruce normally would.
The film goes to on to show that this time, it's Bruce's interaction with Superman that ultimately saves him and brings him back from the brink of darkness. The "Martha" moment is what causes him to realize the "monster" he had become, and Superman's actions and death are what lead to another reversal within him, as he reverts back to a more hopeful individual who believes that "men are still good" and that people like himself and Wonder Woman can make a difference.
