Sometimes, a screenwriter is too close to a deadline/too low on vodka to worry about minor things like character development or plot coherency. But never fear! Dedicated fans are always ready to fill in the blanks with theories of their own that, in many cases, are strangely convincing.
Are any of these actually true? Maybe! Do most of them improve the movie? Absolutely.
WARNING: Massive spoilers ahead.
Batman v Superman: Batman Is So Kill-Crazy Because Robin Turned Into The Joker
If there are three things everyone knows about Batman, it is that he does not use guns, he does not kill, and he could absolutely be doing better things with his money. Well, in Batman v Superman, two of those things go out the window, together with all the bullets that Batman shoots from guns into his enemies.
"Haha! Why didn't the police think of this?!?"
This very convincing Reddit theory argues that Batman's sudden lax attitude towards killing is due to Batman having a mental break, brought about by his second Robin becoming the Joker. That's right, it's possible that Jared Leto's character was once Batman's apprentice, but was tortured to madness by a bad guy (the original Joker?). Let's look at some facts:
In the film, Robin's costume was marked with comments from the Joker, suggesting that the clown prince of crime killed him. But maybe his death was only metaphorical because where Robin died, the Joker was born? You know, like how Obi-Wan insisted Vader "murdered" Anakin Skywalker.
And then spray painted all over his Jedi robes.
After all, Batman does comment on how few people stayed good after 20 years in Gotham. That level of cynicism can be expected after watching his apprentice morph into everything he hates. Also, it does sort of look like Leto's Joker has tattoos covering up the spots where Robin's armor was shot (right shoulder, below the ribcage).