It doesn't need to be 18+. The only thing that would change would be the blood. It doesn't need to be M to have a great story. Look at the recent Call of Duty's. All of them have had shit stories, except for BO1. A game doesn't have to be overly violent or sexual to have a good story.
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BO was rated m and its story was magnificent. I'm pretty sure shattering glass in someone's mouth is m-worthy.
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It limits what the characters can do and say, and have you even played he recent cod campaign's? Let me geuss no. advanced warfares campaign was decent and so is black ops 3 you clearly don't know how game development works, games that have great stories are almost always mature/18+ rated.