The Slytherins who were redeemed were personally redeemed, and became separated from the house. I remember when I'd sometimes mention DD's odd reaction to Harry's "OMG, I could have been a Slytherin and I'm so ashamed!"--where he just reassures him he's a Gryffindor and the sword proves it. I remember one time somebody explaining to me that Dumbledore really isn't agreeing with him that Slytherin is a bad thing to be. But in DH I thought it was very much proved that it is a sign of poor character--when DD compliments Snape's bravery in GoF he even does it by suggesting that maybe they Sorted him too early. The only time Slytherin "traits" (if they have them) are admirable is when they turn up in non-Slyths. They seem basically incapable of the kinds of choices for good that normal people make easily.