Letho spoilers and Radovid spoilers follow:
Why did so many people let Letho live in Witcher 2? Secondly, given that the Witcher 2 made such a big deal about Witchers not supposing to operate as assassins, did anybody else thing long and hard about taking on the Radovid mission?
Much like most characters in Witcher, there is no clear bad and good. He was used because there was a clear goal in mind for himself and his school. Plus he did things in his past that had a direct positive impact in the life of Geralt. He isn't a cut and dry "bad guy".