I heard somewhere that the reason Salazar Slytherin was so opposed to allowing mud-bloods to enroll in Hogwarts was because around the time Hogwarts was founded it was the 10th-11th century. During which time witches were commonly persecuted by muggles and often slaughtered without consideration. Apparently Slytherin felt that magic practitioners who were half muggle could potentially be used as double agents by witch hunters and the likes to infiltrate Hogwarts and put pure blooded witches and wizards in danger. Hence why he built the Chamber of Secrets and had the basilisk target mud-bloods. It was a contingency against such a potential threat.﻿IF TRUE =