That may have been the case in Harry's time, but don't forget that Hogwarts was already over a thousand years old when Harry was accepted there as a student, and that we don't know much about Hogwarts before Voldemort came there as a student.
In the Fantastic Beast series we see of course a Hufflepuff in the protagonist role (Newt Scamander), but his friend Leta Lestrange, was definitely not an evil woman, and yet she was a Slytherin.
Or course, since Gryffindor and Slytherin where known to be the worst of rivals, and Harry as main protagonist of the series being a Gryffindor, the Slytherins easily get an evil label since the original books were all written of at least 90% from Harry's point of view, so any opinions cast in the books are most of all Harry's opinions. This easily clouds the readers mind about Slytherin house.
What is lesser know as that Rowling did include Merlin into the HP franchise as a Slytherin, and this while the order of Merlin is most of all given to people to those who did things that could improve the relationships between muggle and wizard.
And when it comes to the books, perhaps take a look at Horace Slughorn. He is a Slytherin and the way he "collects" students is truly worthy of a Slytherin. Bringing hopeful students (including Harry and his mother Lily) to the top, yet staying out public view, but still benefiting from being the one who brought these students to their important positions, also expanding his network with important organizations in the process. Very clever and cunning, but I really wouldn't deem him evil. Yes, he "uses" students to get better himself, but in the same time, the students themselves also benefit in a great way. The remorse he shows for explaining to Voldemort what a Horcrux is and how to make one, also shows he is not a bad man.
So in the end Rowling did show that Slytherin house is not that much a House of Evil as you might initially think.
