Auto-translated
Well, firstly, Kosha couldn't kill anyone today, only maim them. Secondly, we should definitely clarify with Nebo, but it seems to me that unsuccessful assassination attempts aren't recorded (in the case of innate resistance, for example). Is that right, Nebo?
Thirdly, you're delving very deeply into philosophy. Let's just say that shooting someone with a machine gun isn't exactly good feng shui ;) It's stated that big means big.
So, we have a murder by HECU, and by Zen. There's no murder by the X race, by the black operatives, or by the city. It's quite possible that the first two were just unlucky (or Kosha was sent), and the city was just sitting there. I think it all adds up.