Slughorn, Slytherin, and Second Thoughts
There's some Harry Potter spoilers in here, but it's been two weeks. I'll try to keep the serious spoiling to a minimum.
Early in Half Blood Prince we meet Horace Slughorn, a professor Dumbledore coaxes out of retirement to help fill that recurring vacancy issue. (Defense Against The Dark Arts teachers have roughly the lifespan of Spinal Tap Drummers, though I'm not sure whether spontaneous combustion or having Voldemort sticking out the back of your head is worse.)
Slughorn was once head of Slytherin house, thus was once a Slytherin himself. Given that Slytherin's best known alumni are Lucius Malfoy, virtually evey Death Eater, and, oh, one Tom Marvolo Riddle, you'd think Slughorn would be mean and evil and nasty like his successor as Slytherin head of house, Severus Snape.
But Slughorn illustrates another facet of Slytherin history. He's a social climber, a backstage operator, the living embodiement of "it's not what you know, it's who you know." In short, a political schmoozer. He displays a slight prejudice against Muggle-borns, but nothing like the "m-word" vehemence of the Malfoys. And he immediately latches onto Harry, drooling over his celebrity and connections.
The Sorting Hat, voice of the Four Founders, NEVER describes Slytherin as "evil" or "dark." "Those cunning folk use any means to achieve their ends," it sings in Philosopher's/Sorcerer's Stone. That might make the Slytherins the Omega House in some Hogwarts meets Animal House sense, but it doesn't mean they're all the spawn of Satan.
Well, OK, Draco is... but perhaps in the post-Voldemort era (we hope), Slytherin will return to something no nastier than the bully-jocks rather than the Death Eaters Club.