Considering he was best friends with Godric Gryffindor for years, I don't think he was pure evil, however he was prejudice against muggleborns. This may not have been completely unwarranted considering the time. Wizards and witches were all considered evil by many people and one of the reasons Hogwarts was established was safety for them. Muggleborns may have posed a security risk in Slytherin's view and that is may have been the reason why he created the chamber of secrets when the construction of the school was being planned.