(or, even this could have been worked into an AU in which Salazar Slytherin was not eeeeevil and irrationally bigoted)
So, lately, revelations on Pottermore have been giving us bits and pieces about the Founders: most pointedly, things that counteract the moral ambiguity introduced by DH (in which it was stated that Godric Gryffindor stole his Sword from the goblins, and that Gryffindor House may not have been the bastion of heroism that the previous six books had presented it as.) But Pottermore sweeps this away entirely, restoring the status quo of “Gryffindor heroes, Slytherin villains.” Notably, it sweeps away any possible interpretation that anti-Muggle sentiment might have made sense in the face of persecution, stating that
“Slytherin’s discrimination on the basis of parentage was considered an unusual and misguided view by the majority of wizards at the time. Contemporary literature suggests that Muggle-borns were…accepted [and] often considered to be particularly gifted”
and implying that persecution wasn’t a major issue at this period in time (suggesting that persecution/witch-hunts were much more prevalent in the immediately pre-Statute period - that the 1600s were a comparatively traumatic time that caused a major shift in opinion in the magical community, thus sweeping away any possible idea that Slytherin’s ideas could either have been common or justified in his own time period.)
This isn’t surprising - JKR’s pro-Gryffindor preference is obvious in the text, and my meta on my issues with Slytherin’s treatment remains by far the most popular post I’ve ever made on Tumblr - but it is disappointing, since having a more complex view of the Founders would have been more interesting, and it’s difficult to imagine how the progressive Gryffindor and regressive Slytherin were the best of friends (as the Sorting Hat describes them at the time of the school’s founding.) But what I did find surprising was, when I expressed disappointment and proposed an AU history that would have been compatible with canon pre-Pottermore, I got people asking me how it was possible I could come up with any alternative interpretation - after all, Slytherin brought a basilisk to school, and we knew from the Tales of Beedle the Bard and general background information that Wizarding persecutions were never really that serious or dangerous, to the point of some wizards and witches allowing themselves to be captured and burnt at the stake for amusement value.
Pottermore’s information, indeed, makes it impossible to hold a canon-compatible alternative view of Salazar Slytherin. But I would argue that, using only the text of the series, it would have been possible to establish a Salazar Slytherin who wasn’t batshit insane - someone who did actually have a plausible reason for installing a basilisk in the Chamber of Secrets (and one that didn’t involve murdering his own students, Muggle-born or not.)
This ties in with that plausible alternative wizarding history I articulated before: that there is a plausible AU in which Salazar Slytherin was actually rational. (Bigoted, yes, but in such a way that made sense for his time period - and which would have been distorted by members of the pure-blood movement in the 1600s, since per the information on Pottermore, the pure-blood movement was very much a social and political movement tied to that time period, with the rise of the Statute of Secrecy and families like the Malfoys looking to exploit public sentiment and the concept of blood status for their own gain.)
Since this qualifies as a long post, I’m sticking the rest of this under a cut: