It\'s quite simple, really.
When you\'re offering such services, they become more common. something that was distant from a culture, now becomes a part of it. At first you have only shady characters using those, but things are changing. When something is common, people are more certain to reach for it. Assassination stops being seen as something so utterly wrong. New ideas emerge. Murder the tyrant, discourage the merchant who became a strong competition for you, or find out how come he\'s so good at everything. With time more and more characters would use such services and the culture would change into more dark one. What would be seen as \'evil\', might change quite a bit from the real meaning of this word. New generations, raised up in such environment would have disturbed view on what is good and what is evil. They would see assassination and spying as something much more normal than what it was considered by a generation before. The services would gain more and more costumers, eventually turning Yliakum into a dark world, where trust is no longer present in such a high form. Friend would spy on friend, just to make sure he doesn\'t plan to murder him.
Influencing culture is the best way to gain control in a long run. You can use force only when your power is amazing or when you want fast results. You have to keep in mind however, that people will unite against you. The former way would rather bring you followers.
From economic point of view, you\'d have a stable, self-enlarging market, where more and more types of characters decide to use your services. If dealing with shady characters only, you don\'t influence anything. There\'s already a market in a certain form and it won\'t change in any way. That is unless you could come up with new services. But even then, as long as you limit yourself to dark characters, you don\'t influence world in any stronger way, nor do you join any stable market.
Even when good guilds might decide to go on a hunt for evil, the Whisper would remain much more safe, seeing as it might be accepted by a lot of commoners. The will to fight it wouldn\'t be strong enough.
And thus, in case of good side taking a lead, there\'s still one source of evil that wouldn\'t be really hunted to death.