Letho feels to me like another character who believes that the end justify the means. All he wanted to do is restore his school, that is why he accepted the job as an assassin in the first place. Not that he could refuse anyway. He is, despite his appearance a very smart man, and those who are fooled by it, pay the price, just like Sile and the Lodge. I find it quite unique that a Sorceress like Sile, one of the most intelligent people of the witcher world, got duped by a "simple" witcher. Get's me thinking, that there is a grain of truth, in what Letho had told her, what it feels like being a witcher for him, even though he acted tough when he was telling Geralt about it.
There is really no reason to kill him, unless you play Geralt as the ultimate Temerian patriot, but I doubt Geralt's "character" would go there.