In the Harry Potter universe, Slytherin House has a bad reputation to put it lightly. After all, Lord Voldemort, chief nemesis of the series, is a Slytherin. The founder of the House, Salazar Slytherin, held personal views towards muggles — non-magical folk — that the series rightly condemns. But to judge the House simply by that standard is intellectual laziness and does not do justice to the series. Hence the need for a defence of one of Hogwarts' four great Houses.
As an aside, I have a vested interest in this one given that Pottermore sorted me into Slytherin not just once, not just twice, but three different times.
First, it is charged that all Slytherins are evil. This is demonstrably false. Professor Severus Snape, whom Harry himself described as ''the bravest man I ever knew'', was in Slytherin. Was his not the love that kept Harry alive? And, let's not forget, Harry's own son — Albus Severus Potter — is himself a Slytherin.
Second, it is charged that all villains in the series are Slytherins. Nonsense. Peter Pettigrew, who betrayed Harry's parents to Voldemort, was a Gryffindor. So much for the stereotype that all Gryffindors are paragons of honour and bravery. And let us not forget another fellow Gryffindor — Gilderoy Lockhart — that epitome of failure.
Third, there is the argument that no wizards of good repute belong in Slytherin. Rubbish. Merlin — one of the most famous wizards of all time — was a Slytherin. The Order of Merlin, bestowed upon those witches and wizards who perform acts of greatness for the wizarding world, testifies to Merlin's status. In fact, despite being a Slytherin, Merlin's was the contention that there was no need for animosity between magic folk and muggles.
Fourth, there is the argument that bravery does not belong in Slytherin. Again, nonsense. Yes, Horace Slughorn has skeletons in his closet, but is it not he who leads the final charge at the Battle of Hogwarts and duels Voldemort himself? Or what about Narcissa Malfoy who deceives the Dark Lord and, in so doing, keeps our hero alive?
And on the topic of Slytherin and muggles. Granted, Salazar was deeply suspicious of muggles and believed that magical education ought to be restricted to pure-bloods. To quote Ron:
"I always knew Salazar Slytherin was a twisted old loony, but I never knew he started all this pure-blood stuff. I wouldn't be in his house if you paid me. Honestly, if the Sorting Hat had tried to put me in Slytherin, I'd've got the train straight back home..."
But one cannot condemn the entirety of the House that happens to bear Salazar's name; to do so would effectively be to condemn no less than a quarter of Hogwarts. And, as the series demonstrates repeatedly, suspicion towards muggles is in no way unique to Slytherin House (Arthur Weasley, after all, appears to hold some strange fantasies).
And what of the character traits inherent in the House? To quote the Sorting Hat:
"Or perhaps in Slytherin, you'll make your real friends, those cunning folk use any means, to achieve their ends."
Ambition. Cunning. Resourcefulness. Slytherins pride themselves on these qualities, each of which is innately part of being human. Harry himself exhibits many of them. To their credit, Slytherins utilise said qualities to achieve their aims. Little wonder, then, that by the time Harry arrives at Hogwarts, Slytherin had won the House Cup six times consecutively. They can't be that bad then. As the Sorting Hat makes clear to Harry:
"You could be great, you know, and Slytherin will help you on the way to greatness, no doubt about that."
Calling Slytherin the "evil" House is ridiculous. The Harry Potter universe is much more complex and multi-faceted than that. Those who castigate the entirety of Slytherin commit nothing less than a heinous act of self-perpetuating prejudice. Heroes and villains exist in all of Hogwarts' four ancient Houses. Naturally, every House thinks it's the best (Slytherin really are, though), and every House is as much as a part of Hogwarts as the other three.
Slytherin just happen to be less pretentious than Gryffindor, more complex than Hufflepuff, and, well, more exciting than Ravenclaw.
So, found out that you're in Slytherin? Welcome aboard!