The answer was held within a sacred charge, passed down from Head of Slytherin to Head of Slytherin, since the time of Slytherin himself: protect the children.

Slytherin was a half-blood; child of a snake-yokai and a witch. His sire had died just after his child’s fifth birthday, after fighting off the human priests whose God demanded that they kill the ‘demon spawn’; a plump, green-eyed boy with a lisp and soft, acorn-brown locks. His bereaved mother had returned to her family who, fearing the eminently powerful child, had, thereby, treated Slytherin with cruelty and neglect.

So, unlike Gryffindor, the treasured child of indulgent parents, who could not believe that any heart could be cold enough to allow the neglect, let alone abuse of a child, unlike Ravenclaw, whose head was, aptly enough, so deep in the clouds that she couldn’t touch bottom with a long stick, unlike kindly Hufflepuff, who doled out honey cakes and sympathy but was too weak-stomached to bring the perpetrators to justice, Slytherin was the only founder who could see the bruises beneath the glamour and would seek vengeance.

Let the angels watch over children whilst they slept, the demon would destroy their nightmares. Slytherin, famed for his deadly vendetta against those who preyed on the vulnerable, came to be known as ‘the Basilisk’ for those who fell under his gaze would die as surely as if they looked into the dread eyes of the king of snakes.

Slytherin’s paranoia over the safety of Hogwarts had lead to many an argument between himself and Gryffindor: Slytherin protested that muggleborn children must become wards of Hogwarts at birth, lest their parents destroy them when their gifts manifested. Gryffindor could not bear snatch babes from the cradle: of course their parents’ love would be stronger than their prejudices. Slytherin wished that all muggleborns and their close kin be placed under a tongue-binding charm, so that their children’s powers and Hogwart’s existence remained hidden. Gryffindor thought it the height of barbarity to practice magic against defenceless muggles.

Eventually, Slytherin left in high dudgeon, leaving his position, the title of ‘Basilisk’, the duty and the book to his son. He also left a siege weapon, lest his worst fears should come to pass: spellcraft, while highly effectual in single combat, was useless in a melee: the muggle army could wipe out Hogwarts in a day. Therefore, Slytherin, whose sense of humour was keen, hatched a basilisk egg and hid the tiny snake within the bowels of Hogwarts. The basilisk, which could kill a thousand men with a single glare, was a necessary safeguard.

For the last few centuries, however, the Heads of Slytherin had not the gift of parseltongue but, nevertheless, still bore the title of ‘Basilisk’, the duty and the book: the Basilisk was sworn to seek out the abused, the broken, the wretched and shield them from their tormentors.

Slytherin may not have had the monopoly on abused children but, in Severus Snape, abused child, Deatheater, Potions Master, Duelling champion, those children had possibly the best protector since Slytherin himself.