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.
Contrary to what you and others say, the infiltrator is unquestionably presented as a ninja-class. Infiltrators hunt bounties. Infiltrators assassinate militia. Infiltrators break into tax offices and steal money. I can't think of any aspect of the infiltrator that is not ninja-like. It is therefore consistent to expect the infiltrator to also assassinate nobles; not merely stab them in the leg to keep them in bed for a few days (by the way, how is
that not 'ninja'-like too?).
It is far more absurd to believe that, out of thousands of attempts that have occurred to date, not one infiltrator has ever been successful. Sure, you could play some of these failures off by saying "haha, this wound I have inflicted upon my victim will serve as a warning," but it becomes contrived when you have to give the same or similar reasons for thousands of other incidents. How, exactly, does it make sense for these thousands of failed assassinations to have occurred, with thousands of bounties paid out to them?
Essentially, the problem with the infiltrator is that it is not compatible with the game's setting. You deny that "ninja's" can exist in this setting, yet the infiltrator is nothing but one. You do not want player death, yet you add a mechanic which should logically lead to it. On the one hand, you want to restrict these things, but on the other, you add a class which which does not work without them. It's a case of failure to compute, logic.
