So while I disagree with Cannoli's cynical view of humanity, the Shadow operates by steps. Obviously you can have exceptions like Semirhage, but typically you start off with Darkfriends with minor tasks to fulfill, to rope them in. You give them illegal stuff to do, but things that aren't too difficult for the Darkfriends to justify for themselves. Then you move up to more truly immoral stuff, and by the time the Darkfriend's sense of morality starts acting up, it's too late and he's thoroughly ensnared, like Ingtar.

It's easier with Aes Sedai, because as I mentioned earlier, the kind of people willing to make the journey to the White Tower is often the kind of people who would be attracted by the Shadow's promises. I mean, in many countries, Aes Sedai are seen as the next thing to the Dark One anyway.