Batman: Arkham Knight is the first game of Arkham series to receive an age rating "Mature" ESRB body, considering it suitable only for major players of 17 years, while other chapters of the series were "Teen" from 13 years. The description explains the classification describing some of the most violent scenes of the game.
The description mentions initially traditional actions of Batman as villains face with the most varied blows, causing pain and screams and submissions in slow motion. However, it also states that players can shoot unarmed enemies and even hostages, although the ammunition Batmobile not be lethal.
What apparently secured the higher age rating are two torture scenes, one in which is used the wheel of a vehicle (not necessarily the Batmobile) and another in which someone is tortured in a table bloody operation. There is also reference to some hostages, bound and helpless, being murdered in cold blood.
There are still details related to sex, such as striptease homes with words "Live Nude Girls", "XXX", and strong language during the dialogues. But none of this is so problematic for age rating as the torture scenes.