In the movie, McG explains, in a quite dramatic fashion, that the chamber is the home to a MONSTER! But in the book, Binns, who is probably much more older than Minnie, doesn't put much stock on those stories.
We are also told Salazar didn't want muggleborns in Hogwarts, and later on he had a fight with Godric and left the school.
But was Salazar Slytherin really a racist? Or were his ideas based on strategic reasons?
The most reliable source of those days is the Sorting Hat, as he was there and he has an echo of the founders' brains in it.
"For were there such friends anywhere
As Slytherin and Gryffindor?"
Fact #1: Godric and Salazar were something like best friends.
"So Hogwarts worked in harmony
For several happy years,
But then discord crept among us
Feeding on our faults and fears."
Possibility: It was an outsider force that caused troubles, it is possible Salazar maynot be the source of the problem, as later the Sorting Hat draws a parallel to the current situation:
"Oh, know the perils, read the signs,
The warning history shows,
For our Hogwarts is in danger
From external, deadly foes"
Fact #2: Salazar left. Nobody else but Salazar...
Strong Possibility: All of the founders were isolationers. They had no arguments about the need to hide wizards and witches from muggles. Hogwarts is built in a far off land, and was built in a time that muggle transportation was not advanced, broomsticks didnot exist or very slow, floo powder and Aparation maynot have been invented yet.
Here comes the theory: All four founders agreed that the existince of the muggle world should be hidden from wizards. In my opinnion, the reason Salazar didn't want muggleborns in Hogwarts is because it would risc the exposure of the wizard world. When you accept a muggleborn wizard to Hogwarts, that wizard's parents, siblings and maybe close friends will have the knowladge of the existence of magic. Then it makes sense not to let muggleborns attend. A counter point can be made by telling; by accepting muggleborns to Hogwarts, you can train them and there won't be any accidental magic going around. Not accepting them could be just as dangerous. If you accept them, at least you have some control over the situations.
So what was the fight about? I believe, even at the time, there were wizards who didn't want muggleborns, and not because of their bloodline. From a political point of view; when you pull people from their world and train them in the ways of another, you take most of their chances at functioning at their previous nature. So, that meant muggleborns would have to take a piece of the wizard cake. Some didn't like it. They used Salazar's logical ideas to convince people. And somewhere along the lines, things got awry. Then after Salazar left, the logical argument was gone and all that was left was the terror. Salazar, by leaving, made a statement: "I do not approve of your violant ways." Pay attention: fights ended when Salazar left, Sorting Hat makes that pretty clear. This was a man once best friend of Godric. What he did was noble. Hogwarts was his baby, it was an ideal of his. If he really was a dark wizard who saw he was losing, don't you think the other three would have destroyed everything that resembled his dark ideologies? After all, that's what the winning nations did through history. On contrary, the others are heartbroken because of his departure.
So where does the Basilisk fit in? Simple. It was one of the ways to protect the school. That's why it was caged. The giant spiders didn't come near it, did they? "The chamber can be opened only by the heir of Slytherin" comment is horsecrap; Harry opened it. All it took was a parselmouth. If Salazar intended to kill muggleborns, why take so many precautions to keep the basilisk away, and make it wait for a heir for a thousand years who may or maynot want to use it?
I don't believe Salazar Slytherin, the best friend of Godric Gryffindor, was a dark wizard who wanted to kill innocent people just because of their bloodlines. He was a man who other dark wizards tried to manipulate by using his logical reasoning to back their war plans. And Salazar's answer to those men was very loud and clear.