Why is Modern Warfare an 18?

Well the majority of the game is a 15. However, there is this one mission in which you have to shoot innocent civilians, and apparently this "dwells on the infliction of pain" which makes it an 18. This mission is actually skippable, so if you chose not to play this mission, it would be a 15.

well first the use of language and the amount of blood that is why and also just for killing in it

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Blood, Drug Reference, Intense Violence, Language.