Most people in the "Harry Potter" fandom tend to still count Slytherin as the "evil house". While I do agree that a lot of so called evil witches and wizards come from Slytherin, but I do not believe that Slytherin creates this evil instead I believe the wizards and witches put here already have a potential for evil based upon their character traits.
Slytherins just seem evil because of their traits. They are prideful and ambitious. They strive for what they want, and sometimes this means that nothing gets in their way. They want to be extraordinary and want to achieve more. The world doesn't view these traits as a negative thing--in fact we view them as leadership skills, so then why do we deem all members of Slytherin as evil? We only see the house through Harry's eyes.
Harry despises the house because of what it has done to his life-- Voldemort, the Malfoys, Bellatrix and even how he viewed Snape at first. These characters ruined Harry's life and they all have one thing in common, Slytherin. With most of your enemies having this common ground why wouldn't you place that stereotype on a house?
Now. Slytherins (like myself) face this stereotype of being evil. But, the stereotype is not true plenty of Sytherin alums in the wizarding world turned out not to be evil but quite the opposite, even Snape. Slytherins such as Merlin, Andromeda Black Tonks, Horace Slughorn, Regulus Black and many more we know even less about have done things for good.
Merlin, the great wizard we know him to be, founded the Order of Merlin which prevents the use of magic on muggles. Someone that fought for muggle rights and never did anything to harm anyone else could definitely not be evil.
Andromeda, though she was raised to be hateful and racist against anyone who wasn't pureblood she rejected her upbringing and married a muggle-born, Ted Tonks. She then aided the Order of the Phoenix and began to help the cause. Her daughter Nymphadora lost her life in the Second Wizarding War and so did Remus Lupin, Nyphadora's husband. This left Andromeda to watch her grandson, Teddy. None of Andromeda's life says evil.
Horace Slughorn, though he is naive, was never evil. He prized his students and would do anything for them including cluing Tom Riddle in on how to make horcruxes. Head of Slytherin house he reaped the benefits of befriending his students, but some he never liked such as Dolores Umbridge. In the battle of Hogwarts he brought reinforcements to help defend the school. He also helped protect students from the Carrows when Snape was Headmaster.
Regulus Black defied Voldemort and tried to destroy the locket which was a horcrux. Anyone who died defying Voldemort isn't evil to me.
So, stop the stereotyping and get to know a person not a label, because Slytherins are not evil just like other houses can be.
To all the Slytherins out there-- remember you are what you want to be not what your people say your house is. Also, remember to find yourself a good Hufflepuff to settle down with!