If you used the accusative form, it would have a meaning similar to 'one of the newspapers'.
Imagine you are a cop. There are maniacs in the city centre just shooting at everybody for no reason. You go there and shoot one of them. You say to your superior on the radio "Bir adamı öldürdüm (I killed a man(one of the men) )". Afterwards you think "Oh my God, what have I done! I have just killed a man! " and say "Bir adam öldürdüm! (I killed a man!). Bohemian rapsody