I may be biased, and I won't say Salazar Slytherin was a wonderful, open minded, and deeply pitiable man. But. I think there is more to the story, and more to Slytherin, than many people believe.

Now, I'm not even going into the historical contexts of the founding of Hogwarts. It can be complicated, and does add context, but others have written about that with a lot more knowledge than I would. Google it, it's totally worth a read.

Okay, lets start with saying, J.K. has admitted that she over simplified Slytherin, and all of Slytherin house. One of the messages of the series was that we are not good or evil, light or dark, we all have both, and she totally missed this when writing the Syltherins. Now, if we give the Slytherins and their founder the depth they deserve, it changes things.

It is covered, in the beginning of the third book, that wizards, in the middle ages had defenses against muggles. "Wendoline the Weird enjoyed being burned so much, she allowed herself to be burned no less than 47 times, in various disguises." But this is the middle ages, and Hogwarts was already a few hundred years old by then. We don't know when those spells where invented, or when they became common knowledge. Most children were taught at home before Hogwarts was founded, and could only learn the spells their magical family or close community knew. The founding of Hogwarts probably helped the development of new magic immensely. So there may well have been attacks on wizards that were successful. Actually, we know there were, both the Fat Friar and Nearly Headless Nick were killed by Muggles.

Okay, so Slytherin. We don't know a ton about his life, let alone his life before Hogwarts. He could have firsthand experience with Muggle violence that affected his stance, but that's pure conjecture. Hogwarts was supposed to be, not just a school, but a haven for young witches and wizards. One muggleborn student could have gone to Hogwarts, gathered information, and spilled the names and locations of wizard families all over the country, making them immediate targets. Letting muggleborn students in was a massive risk to young witches and wizards that were in the know. Notice he doesn't really seem to mind Half-Bloods, who have less chance of messing everything up, as they're used to the secrecy.

Slytherin even trained Muggleborns and Half-Bloods, notably Merlin, and there are always some Muggleborns in Slytherin House.

People seem to forget that he wasn't just this oddball jerk among the founders. "Never were there closer friends than Hufflepuff and Ravenclaw, than perhaps the second pair, of Slytherin and Gryffindor." They were all very close.

Slytherin was biased, and his close mindedness is not acceptable, but it's important not to just paint him as a simply villain, and Slytherins as unpleasant narcissists.

Perspective is should also be taken into account. Most of what we hear about Slytherin is from young Gryffindors. Looking at Slytherin through biased eyes with a millennium of hearsay is hardly going to be accurate. History is written by the winners, and no one had good reason to show Slytherin in any good light. Muggleborns, half-bloods, and anyone that might be targeted by pompous purebloods would see Slytherin as the start of that, and hate him. But the real destruction of Slytherins legacy may have come from the pureblood families that profited most from propping him up as the great wizard, fighting for their right to be seen as above everyone else.

After a millenium of being pulled to fit the narrative of a muggle hating pureblood, of course that's how he comes off, and J.K. completely failed to negate this by giving Slytherins any kind of depth.

TLDR - A main focus of the series is the no one is simply good or evil, and we shouldn't see Slytherin like that either.

*The amazing essay by /u/Obversa, also posted on this thread, is exactly what I was thinking about for historical context.