Who were some good Slytherins?
Andromeda Tonks - Was disowned by her family for marrying a muggle-born Hufflepuff.
Regulus Black - Not everyone entirely agrees, and that’s okay, but in my opinion, I consider him good because he did leave Voldemort in the end and died, trying to destroy a Horcrux or to let someone else destroy it after he stole it. He cared a lot about Kreacher and ultimately sacrificed his life.
Horace Slughorn - Sure he has a lot of flaws (like picking different students to be part of a special club, some of them not for very good reasons), but the Slug Club isn’t as bad as the Inquisitorial Squad. Overall, he was a good person and made up for his past mistakes, like giving Tom Riddle information.
Astoria Malfoy - Okay, we barely know anything about her, so this is just my opinion that she was good, and I just really like her.
Scorpius Malfoy - I know people don’t really like The Cursed Child, and that’s fine, but Scorpius is just so precious - I can’t.
Albus Potter - Same thing as Scorpius with The Cursed Child, but I really like Albus’s character in it. He had/has his flaws, and he matured throughout the book.
Merlin - I don’t really care much for him, and we really don't know much about him, but I’m just going to put him on here, anyway.
Severus Snape - I see him more as a bad guy who did good than a good guy who did bad.
Narcissa Malfoy - She loved her husband and son and risked her life to save Draco, lying to Voldemort, and because of that, she saved Harry. She still isn’t overall a good person, as she supported pureblood supremacy and the Death Eaters, even though she wasn’t one herself (until she decided to just save her family - at that point, she no longer really wanted Voldemort to win, because all he was doing was torturing them).
Lucius Malfoy - Same as Narcissa, only he was probably worse (and was an official Death Eater), and he didn’t lie to Voldemort, but he still did some things to save Draco.
Draco Malfoy - He was a bad person and never really fully became good by the end of the series (although he could afterword, I want to use mostly only canon information right now). Like his parents caring for each other and him, he cared for them, and a lot of what he did, good and bad, was for them.