In "Tennessee's Partner" why are the characters not evil villains in spite of some of their actions?

According to the Forgotten Realms Campaign Setting book, Red Wizards are the usually notorious and nefarious spellcasters who are the ruling class in the inhospitable but well-inhabited country of Thay. The Red Wizards are slavers, demonologists, magical experimenters and scholars. They constantly scheme to bring down their neighbouring nations of Rashemen-where they are thwarted by the Witches of Rashemen; Aglarond-where they are turned back by the Simbul of the Seven Sisters; and Mulhorand-where the people of Thay originated.