Salazar Slytherin is an outspoken character in the Harry Potter series. He is one of the four founders of Hogwarts who founded the House of Slytherin and the ancestor of Lord Voldemort, and is thus the posthumous Bigger Bad of the franchise.
Salazar Slytherin was one of the first recorded parselmouths (wizards that have the extremely rare ability to speak to snakes). When he founded Hogwarts with the other three founders, he chose students that show traits of ambition, cunning, shrewdness and determination.
Salazar Slytherin wanted to be more selective of the students admitted to Hogwarts. He thinks that the school should only accept pure bloods and not Muggle borns because he did not trust Muggle borns. He tried to sway the other founders to agree with him but they refused, especially Godric Gryffindor. This caused Salazar Slytherin's friendship with Godric Gryffindor to break and resulted in him leaving the school.
Salazar Slytherin is known to have been the creator of the Chamber of Secrets. He created it in a hidden part of the school. He managed to breed and keep a Basilisk in the chamber. He would later use the basilisk to kill all of the Muggle Born witches and wizards that attended Hogwarts. The other founders did not know of this and probably were not aware that it was Slytherin's doing.
It is unknown when or how he died but it is known that his portrait is still at Hogwarts and his locket was made into a horcrux by Voldemort.
Many Harry Potter fans consider Salazar Slytherin to be a Dark Wizard because he wanted to purge Hogwarts of all the Muggle borns and because he is a parselmouth. Another reason could be that many Dark Wizards and Witches came from the house of Slytherin. Even though the author J.K. Rowling has not stated whether Salazar Slytherin is evil or not many of her fans believe that he is.
Voldemort's death may have ended Slytherin's bloodline.