My theory is the basilisk was Slytherins last ditch protection for Hogwarts, like how Gryffindor left the sword and Hufflepuff the ROR. The whole purpose of the basilisk was twisted by Slytherin's descendants such as Tom.
Like it’s really common in history for things to get mixed up. For example, the quote “let them eat cake” is often credited to Marie-Antoinette but she never said that, some other princess did. It's completely possible (really likely actually) that at some point in time there was a smear campaign against Slytherin and it stuck.
I personally would add something rather than “change”. I just wish Harry Potter (as a series) had much more character arcs/development since the only major arc in the series is Neville’s (and considering the series takes place over 7 years, it’s so incredibly frustrating from a writing point of view).
Like, you know something is amiss when you can remove a character, that an entire book was named after, and not change the plot in any significant way. (The character is Sirius by the way. He offered nothing really to the plot that couldn’t have been filled by another character and it’s such a shame he was so underused by JK Rowling)
The Shadowhunter Chronicles perhaps?