Summary
- Zack Snyder explained Batman's rule-breaking as essential for the character's relevance in DCEU.
- Batman's debut in Batman v Superman faced criticism for plot, tone, and the breaking of the Dark Knight's cardinal rule.
- Snyder defended Batman's actions in the DCEU movie, believing the character would otherwise become irrelevant.
Zack Snyder has defended the fact that he broke Batman break his cardinal rule was enough for many viewers to write it off entirely.
Zack Snyder has, however, defended his choice to depict Batman's murderous intent in an interview with Joe Rogan (via YouTube) where he stated that purposely precluding Batman from situations where he may have to kill someone makes Batman, a character that he also loves, irrelevant. Citing precedents set in DC Comics' Dark Knight Returns - a scene he replicated in Batman v Superman: Dawn of Justice - Snyder stressed that purposely removing him from situations in which he might have to kill diminishes his character. Seeing him make difficult decisions in a no-win scenario, however, is more compelling and real.
"They go like "well don't put him in a situation where he has to kill someone." I'm like, well, that's just like...you're protecting your god in a weird way, right? You're making your god irrelevant if he can't be in that situation. He now has to deal with that. You know, if he does do that, what does that mean? What does it tell you if- does he stand up to it?"

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Why Snyder's Batman Killing Was So Controversial
Snyder's conversation with Joe Rogan was prefaced by a discussion of the zeal demonstrated by fans when it comes to dealing with comic book characters. The backlash that Snyder received from changing this facet of Batman's identity was both widespread and unsurprising, given that Batman's no-kill rule is so intrinsic. Even the original Dark Knight Returns comic that Snyder references comes with ample debate over whether Batman actually killed the mutant antagonizer, despite the looking self-evident. The Dark Knight Returns animated adaptation also retcons the comic book by having Batman knock the character unconscious rather than shoot them.
As a superhero without superpowers, Batman's ability to stand up to the supernatural comes with his impeccable intellect and awareness. Like Star Trek's Captain Kirk (which Snyder references in the same interview), Batman's ability to turn a no-win scenario on its head thanks to a level of preparation and skill that no regular person could have fathomed is precisely how he can implement his no-kill rule regardless of the circumstance. Snyder chose, however, to explore a more realistic interpretation of Batman, which may have been controversial, but has its merits nonetheless.
Source: YouTube