Slytherin was home of many Dark Wizards: that is a fact. Slytherin was also the home of many Light Wizards: that too is a fact, albeit a much lesser known one. Just because Lord Voldemort happened to be a Slytherin, people seem to think that all Slytherins will turn out to be prejudiced megalomaniacs. I do not care what Voldemort's excuses are; it is not right to torture and kill muggles and muggleborns – I refuse now to use the term 'Mudblood', as I would be being hypocritical if I did.
Yes I, Severus Snape, am nothing more than a half-blood. I used to be ashamed of that fact in much the same way as the Dark Lord was ashamed of his muggle father. Now I, unlike the Dark Lord, am no longer ashamed that muggle blood runs through my veins. It took my father's death to show me that I should not be ashamed of whom I am, but before that I had to play the cold-hearted, pureblooded Slytherin bastard.
I too used to be a Dark Wizard; I still am, I suppose, as I still serve the Dark Lord, albeit as a spy. But more so than that, I am Dark because I could do nothing to prevent my students from turning to the Dark Road, the road that I myself mistakenly took as a young man.
In truth, there is a reason that I took the Dark Road, and it had nothing to do with the fact that I wished to be Dark; no, it was very much the opposite. Picture this; a young man is lost within society. He does not know what will become of him. All others tell him that there is good and evil, and that he is the latter, simply because of his school House. Then someone, the Dark Lord, says that there is no such thing as good and evil, only power, and that he will have power if he joins the Dark Lord's ranks. What should he do? Accept the road of the Dark, be accepted for who he is and not shunned away? Or does he say no, and end up murdered and alone, just another nameless, evil Slytherin? What do you think?
There is always a difference between what is right and what is easy, but what would an evil Slytherin know of what is right in the eyes of the majority of society? We do what we think is right, we join him.
You see, it is almost as though we Slytherins have only the capability of turning to evil, but that is not the case. We could have turned out many more Light Wizards and Witches if only we had been given the chance. But we weren't given the chance, were we, and that is why so many of us have and will turn to the Dark Side; we were never given the chance to show that we of Slytherin are simply misunderstood.