Soylent: So nobody noticed that when you save Sile, she tells Geralt where Yennefer is located?
You cant use a candy to pay a heavy loan
GoodGuyA: I personally don't think Letho owes anybody. He paid his debt off to both Geralt and the Emperor through his services. He finally got the chance to sit down with Geralt and reveal every last thing to him. Not only will the emperor be going down soon enough, but Letho really has no business going after the remaining kings. They'll practically kill each other.
He and the emperor just have a bussiness exchange, not a debt or anything
He would destroy the north in any way in exchange for the school of viper to be restored
Frankly, it was all just bussiness.
soldiergeralt: letho's a capable agent of nilfgaard and would prove valuable in their incursion into the north. kill him and deny nilfgaard a very capable assassin.
Snarfinator: Letho was not really working for Nilggaard, Letho pretty much only works for Letho. He was using them like he used pretty much everyone else, to get what he wanted, in this case to help save his witcher school.
Gosh..he worked for the Emperor means he works for Nilfggard >_> the restoration of the school is his pay