The assumption you are making (and I'm flabbergasted that I actually have to reiterate this) is that infiltrators are in any way intended to be able to kill nobles.
The game does not tell you they can do this. The wiki does not tell you they can do this.
If you believe infiltrators should be able to kill nobles, it is because of an image that the players have built up around them. Not because of anything in the inherent design or presentation of the class.
Pretty sure I can find evidence to the contrary snd there have been cases of assassination its juzt that the other player wen t with it by retiring after the attacking roleplaying it as death.
