Don't click any further if you don't want spoilers.
Are you sure? I'm going to put this picture here so you have to scroll past it, so don't go pretending you saw anything by accident.
Here's the rating according to the ERSB:
Rating Summary: This is an action-adventure game in which players primarily assume the role of Batman as he battles several villains spreading chaos across Gotham City. Players traverse a variety of locations around Gotham while battling thugs, examining crime scenes, and occasionally rescuing hostages. Players engage in melee-style combat using punches, kicks, and gadgets (e.g., batarangs, explosives). Enemies cry out in pain when struck, and some takedowns are highlighted by brief slow-motion effects and loud impact sounds. Some sequences allow players to use tank-like vehicles with machine gun turrets and rockets to shoot enemies; a vehicle's wheels are also used to torture an enemy in one sequence. Cutscenes depict characters getting shot (on and off camera) while restrained or unarmed. Large bloodstains/pools of blood appear in crime scenes and in the aftermath of violent acts; one room depicts a person torturing a character on a bloody operating table. During the course of the game, players can shoot unarmed characters and a hostage. Neon signs in a red-light district read “live nude girls” and “XXX.” The words “b*tch,” “gobsh*te,” and “a*s” appear in the dialogue.
As you can see, the mature rating seems to be because of the torture, violence and blood. However, the really interesting line is this: "During the course of the game, players can shoot unarmed characters and a hostage."
That doesn't sound like a very Batman thing to do.
There's a possibility that this could be referring to the non-lethal cannon on the Batmobile; it's used in takedowns, for example. However, the mention of 'shooting' seems to be in a separate context to that. There have been rumours that the gun-toting Arkham Knight was playable and this could possibly explain why 'the player' is shooting people. It's just a guess admittedly, but can anyone else come up with another reasonable theory?