One of the intriguing details in the "Batman V Superman" trailer was the newspaper with the ominous message "You let your family die."
The big question is who sent that hate message? Obviously, someone who's not a fan of Batman and someone who knew him well, to be privy to possibly a dark secret.
The usual suspect would be the Joker, as this is the kind of stunt he would pull, according to What Culture.
The site also points out that "the fact that Bruce would receive such a letter from The Joker obviously indicates that the Clown Prince of Crime is aware that he and Batman are one and the same."
This is a new Extended Universe and the context is very different from the origin story that we saw in Christopher Nolan's "Batman Begins." There might be a totally different connection between Batman and the Joker.
The context of the threat is the Wayne Tower destruction, apparently due to the fight between Superman and General Zod. Was the purpose of the letter to fuel the Batman's hatred towards Superman?
If that was the motivation, then even Lex Luthor could be a suspect. This Lex Luthor could be much different (he is actually Luthor Jr) and there are also separate theories about him.
On the slim chance that it is Luthor, that means he is aware that Batman is Bruce Wayne. The Bruce Wayne- Lex Luthor narrative is a main point in the film as they would probably be allies at first, uniting against the super powered being from the sky.
And who was the "family" that died?
It is unlikely that Bruce Wayne had an actual family. This brings us to the Jason Todd/Red Hood theory, which Movie Pilot put forward.
If it were Jason Todd, he could be referring to his own death (as per The Killing Joke) and the hate message would be connected with the alleged Robin costume. However, if the letter is about Wayne Tower, then it would not be plausible.
Let's see which theory holds up on March 25, 2016.