When and Why Did Batman Stop Killing People?

What is Batman’s one hard and fast rule? No killing. Or, to be even more precise, not using guns to kill people. This has been a piece of his character since the beginning, and it is one principle that he has no intention of going back on. Right? Well, that isn't entirely true. Even in some of Batman's big-budget outings, like Ben Affleck's portrayal in the DCEU, he has killed.