Did Salazar Slytherin actually use the basilisk to kill anyone during his lifetime?
Or did he only hide the basilisk in a deep, dark, dank place?
Prefer answers from canon sources.
In the JKR writing on Pottermore about the Chamber of Secrets, it’s stated that Salazar Slytherin had decided before he was thrown out of Hogwarts that he would keep a basilisk in the Chamber, and either he or his descendants could control it. However, it’s not mentioned that he ever actually did kill anyone with the basilisk either before being thrown out or by sneaking back afterwards.
What is certain is that by the time Slytherin was forced out of the school by the other three founders, he had decided that henceforth, the Chamber he had built would be the lair of a monster that he alone – or his descendants – would be able to control: a Basilisk. Moreover, only a Parselmouth would be able to enter the Chamber. This, he knew, would keep out all three founders and every other member of staff.
The existence of the Chamber was known to Slytherin’s descendants and those with whom they chose to share the information. Thus the rumour stayed alive through the centuries.
- Chamber of Secrets (Pottermore)
From this it seems unlikely that it was known that Salazar Slytherin had succeeded in killing anyone with the basilisk, because if he had, that’d likely be mentioned in the writing. It’s possible that he did and it’s just not mentioned in the writing for some reason, but it’s more likely that it didn’t happen.