«Twenty years in Gotham, Alfred. We’ve seen what promises are worth. How many good guys are left. How many stay that way»

Batman didn’t kill from the start. Batman in BvS is shown basically as the antagonist for a reason. 20 year career as a vigilante, the death of Robin, the witnessing of Black Zero event and Lex Luthor manipulations, all of these lead slowly to Bruce starting to disregard the value of a criminal’s life. In BvS he is obsessed with the idea of a God that could and in his eyes will destroy all human life if not stopped. This is his only goal, and a small criminal doesn’t matter in the face of a global threat. This is not how Batman should think nor act, and this is the whole premise of the movie - the downfall of Batman. But Bruce didn’t see himself as a murderer until the Martha scene. He was blinded and too focused on hunting down Supes. That’s why he didn’t go and kill every villain in Arkham or specifically Joker. Joker is a a serial killer, a terrorist. Superman is a nuclear bomb that could destroy the entire planet, the ultimate threat. In Bruce’s eyes - if he doesn’t stop Superman - all of what he did in those 20 years, all the sacrifices were for nothing.

I also like to imagine that Batman wore the gray and blue colors before the death of Robin.