Information on Salazar Slytherin and the Chamber of Secrets can be found at Pottermore; I've taken screenshots for those without a Pottermore account, which are here and here.
Salazar Slytherin was indeed a Dark Wizard. This is my interpretation of Slytherin, based on the following information provided by J.K. Rowling, as cited above.
Slytherin built the Chamber of Secrets without the knowledge of the other three founders (However, the fact that there were ongoing rumors about the chamber indicate that Slytherin either told one or more persons about the chamber, or he showed it to others directly at some point.).
Even though each of the founders agreed to construct the Hogwarts castle together, each would individually create the four Hogwarts houses and pick where the common rooms and the dormitories would go. However, it was only Salazar Slytherin who went so far as to build what was basically a subterranean homage to himself -- the Chamber of Secrets. Not exactly a modest guy.
J.K. Rowling describes the Chamber of Secrets as being very long and tall, with a massive statue of Slytherin himself that was as tall as the chamber itself was built at the end of the chamber. This, says J.K. Rowling is demonstrative of Slytherin's grandiosity and sense of entitlement and superiority to the other three founders. As well, Slytherin used the Chamber of Secrets to teach students the Dark Arts (Apparently there was disagreement between the founders as to whether or not the Dark Arts should be taught; Slytherin believed they should be and he taught them surreptitiously.). Actually, the Chamber of Secrets started out as merely a secret headquarters for Slytherin and a place to teach his students the Dark Arts, but it ballooned into a secret sanctuary that Slytherin continued to expand as a tribute to his own greatness as a wizard.
The only way to enter the Chamber of Secrets without being accompanied by Salazar Slytherin was to speak Parseltongue to the door guarding the chamber. As we know, being a Parselmouth is considered to be the sign of a dark witch or wizard. We know of three, possibly four, Parselmouths in canon: Salazar Slytherin; Tom Riddle/Voldemort; Harry Potter; and Dumbledore¹. That's a fifty-fifty percent (or two-thirds, if one counts Dumbledore) ratio of dark wizards to light wizards, although Harry was the only one of the four who never had leanings towards the Dark Arts or harbored dark ambitions².
‘Hannah,’ said the stout boy solemnly, ‘[Harry's] a Parselmouth. Everyone knows that’s the mark of a dark wizard. Have you ever heard of a decent [wizard] who could talk to snakes? They called Slytherin himself Serpent-tongue.’ -- Ernie Macmillan, Hufflepuff student, speaking to fellow Hufflepuff Hannah Abbott
Chamber of Secrets - pages 148-149 - Bloomsbury - chapter 11, The Duelling Club
Most importantly, Slytherin created a Basilisk to be used to purge Muggleborn students from Hogwarts; his beliefs that Muggleborn magical children should not be educated at Hogwarts started the centuries-old pure-blood versus half-blood versus Muggleborn -- or, the blood purity -- discrimination sentiments, sentiments that J.K. Rowling has openly compared to those of the Nazis during the Holocaust. Not only did he create the Basilisk, he did so with willful malice, knowing all the while the extreme lethality and danger of a Basilisk, for the sole purpose of killing Muggleborns, and he passed these sentiments down his familial line. Through the Basilisk and his own heir (Tom Riddle/Voldemort) Slytherin preyed on children, children who were hardly capable of identifying an immediate risk of a Basilisk attack or protecting themselves from it. Hermione was the lone exception; the other potential deaths were prevented quite accidentally, the victims ending up being petrified instead of killed. Any wizard that would gladly prey on essentially defenseless children, children whose only crime was that of being Muggleborn, is a Dark Wizard.
‘A rift began to grow between Slytherin and the others. Slytherin wished to be more selective about the students admitted to Hogwarts. He believed that magical learning should
be kept within all-magic families. He disliked taking students of Muggle parentage, believing them to be untrustworthy. After a while, there was a serious argument on the subject between Slytherin and Gryffindor, and Slytherin left the school.’ -- Professor Binns, History of Magic
Chamber of Secrets - page 114 - Bloomsbury - chapter 9, The Writing On the Wall
¹According to J.K. Rowling, Dumbledore understood Parseltongue; it's not one-hundred percent clear as to whether or not he also spoke Parseltongue, for J.K. Rowling has also said that Parseltongue is not a language that can be learned, for there aren't many Parselmouths around to teach it. How Dumbledore came to understand and possibly speak Parseltongue is unclear.
²I know that many readers tend to almost canonize Dumbledore as being the model of purity, altruism, and good intention. Please read chapter 35, King's Cross, and chapter 18, The Life and Lies of Albus Dumbledore, in Harry Potter and the Deathly Hallows to see what I mean by "harboring dark ambitions".