USK would probably be a 12+ rating. There's no excessive depiction of violence as far as I can tell (defeated enemies simply keel over, no blood or gore) and the stuff in the redlight district is rather tame by european standards.
The game wasn't rated by the USK but it would most definitely get a 16+ rating. The lack of gore effects doesn't matter much, what does is that the gameplay primarily involves violence between humans, you can slaughter civilians without repercussions, the story (arguably) promotes vigilantism and everything about the game reeks of nihilism. If it were a first person game it would easily be a contender for a USK 18 rating, even without gore effects (it wouldn't be the first such case, mind you).
Keep in mind that even a game like Defcon
(which is quite old but was released when the USK had already become a lot more tolerant) received a 16+ rating even though there's no actual depiction of violence or even people in there. The depressing worldview and atmosphere combined with "millions killed" as the score unit was enough for them to consider it unsuitable for a 12+ rating.