Presumably Slytherins aren't inherently bad, they just value different traits in a person compared to the other houses. But it seems that in the stories, being a Slytherin equates more or less with being a death eater, or at least a Voldemort sympathizer.
The only exceptions I can think of are Snape and Slughorn, and even Snape was a death eater for while before switching sides.
Are there any other good Slytherins?
Snape and Slughorn are both very flawed, morally reprehensible people.
Snape is a creepy stalker pining for the woman who friend-zoned him. He's a sadist and a bully.
Slughorn is an unethical, narcissistic, greedy dabbler in the dark arts. He's too cowardly to do more than petty, selfish evils himself.
Neither of them is a good person.
– Professor Photon Jul 11 '18 at 17:21