Why in the world would you call an assassin for hire who tried to kill you and is partially responsible for the turmoil in the nest sir.
Maybe becouse he do that for his Witcher school. Maybe becouse
Emhyr var Emreis, Deithwen Addan yn Carn aep Morvudd is the only one of few rules in the game that actuall show some heart for some people. (Books - Ciri, Geralt and Yen, Ciri double) Maybe becouse Emhyr was only guy around that don't let wizards lick his ass and make him their puppet. He is still... very kind guy. He saved Triss. (And that was why I just can't kill him in Iorweth way, after I go save Saskia. Fuck, I would be no different then any other motherfucker in Witcher world.)
How much other characters are 'good guys' in Witcher 2 and Letho is bad? Becouse we look at him from Geralt point of view. Foltest was cool guy, but he also forced Triss to become his adviser, forced Geralt to become his bodyguard.
Vernon and Iorweth are actually both idealist guys that just happend to be on other site of the war. I'm preety sure that they probably would get together if they would have some time to not kill each other and drink some vodka.
This is what Sapkowski wanted to create. This is dark world, full of motherfuckers that would sold their own grandpa for gold coin, people mad with power, wizards that want rule the world behind the thrones. There are only few 'good guys', but every from them have dark sites. It is grey world, with some black elements. Nothing is white, most is grey, some are black.