Until now all the Arkham games have been teen rated. So a little bit grown up, but not (apparently) stern enough to stop older kids playing them. With Batman Arkham Knight though, the series has gone full on 'mature'. The ERSB rating summary makes mention of unarmed or restrained people being killed, plenty of blood and two references to torture. Overall, everything sounds a bit more serious this time around.
Speaking to GamesRadar, Rocksteady's Guy Perkins explains that this is due to the game's "dark themes and content" from the studio ending its 'trilogy' (the polite way of sidestepping Warner Montreal's apparently non-canon Arkham Origins). Perkins says that Rocksteady never aimed to make an M rated game, only that it "set out to tell the story that we wanted to tell".
"It is the end of our trilogy, it has some dark themes and that content is reflected in the M rating," says Perkins. Importantly, though, those themes don't seem to have changed the hero. "Batman hasn’t changed from his unfaltering stance on not killing anyone," he clarifies. "We’ve not changed the rules. We’ve not given Batman guns to go and kill people. The Batmobile destroys tanks but they’re unmanned drones so it’s not really to do with that side of things. It’s more to do with the darker places the story ends up going".
Ultimately, he concludes, it was a decision driven by the creative goals of the team: "Game director Sefton Hill was very keen that this was the story we wanted to tell. It’s the end of our trilogy. It will visit dark some places. That was the story we were going to craft and by virtue of the content of the game we ended up with an M rating. It wasn’t a surprise to us".