JASON TODD!!! NOT JOKER! GOD NO!
Yup, this is the thing that has been argued alot about wheter it is The clown prince of crime the joker or Jason todd the red hood is the one who sent "You let your family die" to Bruce Wayne / Batman
Some people says it's the joker, because of the writing style and the red colored writing, so it's seems like blood, which makes sense for a psychopathic looking joker in the DCUniverse, but, what most people forgot is that Joker, doesn't even want to know the Batman's identity? Because it'll be no fun if he knew his identity, so, the most sensible person to have sent this letter to Bruce Wayne/ Batman is
Jason Todd a.k.a The Red Hood
Jason Todd a.k.a The Red Hood, why? First of all, batman did left him to die because of the fact that batman thought Jason's already dead because of the joker have killed him. More evidence is collected from Bruce Wayne himself, in the BVS trailer he said
"20 Years in gotham, how many good guys are left? How many stayed that way?"