For the first...four or five books, Slytherin = House of Evil. That's undeniable. They are all racist, all the girls are described as being ugly, they cheat at Quidditch... They have no redeeming qualities at all.

Now, in theory, Slytherin House isn't so bad. Dumbledore said in CoS:
"Listen to me Harry. You happen to have many qualities Salazar Slytherin prized in his handpicked students. His own very rare gift, Parseltongue - resourcefulness - determination - a certain disregard for rules."

And then Phineas Black's quote in HBP:
"We Slytherins are brave, yes, but not stupid. For instance, when given the choice, we will always choose to save our own necks."

Salazar Slytherin probably was suspicious of Half-Bloods and Muggleborns because at that time, being a witch was kinda dangerous. But over time his pragmatism was warped into just generic discrimination, racism and hatred.

Yes, people like to say "well the Slytherin we see in the books is post-Voldemort. A lot of the kids are sons of Death Eaters and Slytherin House probably wasn't always like that."

Okay...but since it's THAT Slytherin House we saw for six books, it's the one that matters. And it is demonstrably ugly and hated by every other house in the school.

There were a grand total of 3 good Slytherins in these books - Snape, Regulus and Slughorn.