Context.
Gavilar was not "a threat to the greater good" so far as we know. All we are aware of is that the Parshendii are assassinating a man who has not been proven guilty of anything so that he does not execute an unknown plan. We don't know if Gavilar was doing anything wrong OR if the Parshendii could have handled the problem by simply talking to him, because the clearly decided not to (see Gavilar's guesses at his murderer). So we are back to knowing only that the Parshendii assassinated the king of a nation on the day of a peace treaty with reasons they kept to themselves. They recognize their guilt and the wrongness of their actions, but they consider it worth it. That is expressed in the Eshonai POV, and is the exact opposite of that "honor" definition we have been given. The scope is the scope.
Further into context, we were discussing the Parshendii being "bound to honor" or "honorbound" in a way that suggests they always operate by that method. I think if we can recognize a dishonorable, underhanded assassination, we can consider that possibility false.