Only?
I don't really care which-or-if there is a Robin in this film franchise.
But isn't what you describe exactly what Dick Grayson did.
in fact in his first appearance, sneak behind Batman's back and go after the killers on his own. Pretty much what defined the character of Robin.
Then in Batman#1 Joker's first, again Robin (Grayson) goes after Joker alone, and role-kicks him off a building to to his death!
I'ts only Batman who saves Joker for the "law" to deal with.