A question that probably nobody was particularly asking, but Zack Snyder has decided to answer: why, in particular, did Batman decide to use a kryptonite spear to defeat Superman in Batman v Superman: Dawn of Justice? Christ symbolism, of course.
In Batman v Superman: Dawn of Justice, Batman decides that he needs to kill Superman for the good of humanity, and acquires kryptonite in order to do it. One could certainly argue that a spear makes as much sense as anything else, of course, but Snyder had a particular reason for choosing it.
“Why with all the technology at his disposal would Batman build a spear… here’s why,” the director posted on his Vero account, along with a picture of the Peter Paul Rubens painting Christ of the Cross. The Baroque artwork depicts the crucified Jesus being stabbed in the side by a Roman soldier, as occurs in the New Testament account of his death. (The soldier is not named in the Gospels, but is called Longinus in popular legend.)
Christian themes and imagery have been popular in Superman films since 1978’s Superman: The Movie, which had the superhero’s biological father, Jor-El, claiming that he sent Kal-El to Earth to teach humans goodness. (Admittedly, this doesn’t actually make much sense, since he’s being sent to Earth to survive Krypton’s destruction, but whatever.) Snyder has been particularly prolific with it, however, seen most obviously in the Man of Steel where he talks with a priest and strategically stands in front of artwork.
Fans also recently noticed another bit of Christian imagery from that film: when Superman dies (from Doomsday, not Batman), the wreckage around him creates three crosses. Snyder (who has been talkative on social media lately) responded that this was foreshadowing for his unmade Justice League Part 2; some have interpreted this to mean that three characters would be dead by the end of it.
Just for fun, some random comic book trivia: the spear that Longinus used is sometimes called the Spear of Destiny and, in medieval legend, was believed to have supernatural powers. In old comics from World War II, Hitler was said to have it, and could use it to control any superheroes who entered his territory—thus explaining why Superman didn’t fly to Europe to overthrow him.