As a child, Harry Potter was the absolute most magical thing in the world (no pun intended). I dressed up as Hermione Granger as a 10-year-old, and I was so excited whenever I saw Gryffindor merchandise out in stores. I had taken Sorting Hat quizzes before during my peak obsession (8 years old-10 years old), but I usually got Gryffindor or Ravenclaw and was okay with those answers, because those were the "good" Houses at Hogwarts, and Slytherin was just the evil one. When middle school came around, it was considered somewhat "cool" to be dark and mysterious, and I embraced that a little more, while still keeping the love for Harry Potter. Slytherins are somewhat dark and mysterious in the book and in fanfiction, so I wasn't as quick to judge them as the worst as I would have been in elementary school, but still embraced Gryffindor and Ravenclaw, as I'd already gotten them more often in the past. I wholeheartedly expected to be Sorted into one of those houses when I took the official Sorting quiz released by Pottermore. Imagine my surprise when I clicked the button and it was revealed that I was truly a Slytherin. Younger me would have been upset, but I was actually very happy with my result because I could finally be the different, cool, mysterious one.
This being said, I have had a lot of time since Pottermore came out with this quiz to retake the quiz many times and have since gotten Slytherin the most, so I have gained a bit of insight as to the many different qualities and characteristics of Slytherin other than "evil".
Slytherins are said to be cunning, ambitious, prideful, sly, loyal, determined, resourceful, and clever. Sure, it really depends on how these traits are used, but the same can be said for any of the other Houses at Hogwarts-if you are brave as a Gryffindor, but to a fault, is that really the best thing to be? You can be kind, like a Hufflepuff, but you can be too kind and sacrifice your own well-being for others. Ravenclaws work hard at learning new things, but they can sometimes focus more on learning new things and less on maintaining important relationships. So why is ambition a bad thing? You can better yourself through ambition and determination, and being resourceful and clever can help to get you out of a corner you're backed into. I personally pride myself in being able to think of ways that I might be able to get myself out of a problem or achieve my end goals.
There are many Slytherins in the books that use these traits negatively, most notably Lord Voldemort himself, but even the most loyal of Death Eaters had second thoughts in the end. There are several Slytherins that can be looked back on as having a bit of good in them, such as Severus Snape (I know, he's a cliche example and I'm not saying he's perfect, either-he's problematic at best), who took a great risk to his own safety in order to protect his one love's surviving son, our beloved Harry Potter. Regulus Black, Sirius' Death Eater brother, stole one of Voldemort's Horcruxes, aiding Harry in his quest to destroy Voldemort once and for all. Professor Slughorn was one of the more harmless Slytherins, in that he never sought out to be cruel or vindictive to any character, but rather used people to his advantage for their popularity (which admittedly, isn't really a great way to be, but it's relatively harmless). Merlin himself was a Slytherin, and he's basically the most famous wizard in the Wizarding World. Even Narcissa and Draco Malfoy had their redeeming moments, choosing to not sacrifice Harry to Voldemort when they could have easily been killed if he had found out they were lying. Andromeda Tonks, the mother of Nymphadora Tonks, chose to marry a Muggle-born wizard and abandon all that she had ever known for love. Is choosing love and the right side (albeit a delayed response for most of these characters) evil? Also, there are many characters in Slytherin that we never even hear about, or if we have heard about them, we don't hear much because they are not as antagonistic as the others-in essence, they've sort of chosen neutrality. We can't judge whether someone is good or bad if they don't show what they think.
On the flip side, despite what Hagrid and Ron Weasley think, there are wizards who went bad that were not in Slytherin. Quirrell was not in Slytherin (he was a Ravenclaw!), yet he allowed the Darkest wizard of all time to use him as a host during his attempt to kill Harry during his first year at Hogwarts. And, of course, we cannot forget the most despicable non-Slytherin, Peter Pettigrew. He literally chose the darkness and "protection" from Voldemort's wrath and revealed the Potters' location in order for the Dark Lord to kill one of his best friends and his wife. Even after thirteen years of time separating him from this heinous event, he still returned to the aid of Voldemort to help him regain his physical body and begin his second reign of terror in Britain.
There are also other less-than-savory characters that either has unknown House affiliations, like Barty Crouch Jr., or those who never went to Hogwarts but are still considered "bad", like Igor Karkaroff, High Master of Durmstrang, or Gellert Grindelwald, who went to Durmstrang and was the Darkest wizard before Voldemort came along.
Now I'm not excusing the majority of Slytherin's blatant prejudice and racism in the books. I think it's wrong for them to act that way, and Rowling meant for them to be the antagonists. But there are two sides to every story, and many Slytherins only act that way because they literally had no chance to learn to be any better. I can see that many characters likely only became evil because it was expected of them, or they didn't have the bravery that very few Slytherins have to choose the good side.
Everyone has a black sheep in their family, right? That doesn't mean they're necessarily evil or wrong-they're just different. I believe, in the Hogwarts family, that this can be said of Slytherin.