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No. The primary requirements for House Slytherin are ambition and an aptitude for strategic planning ("cunning," as the Sorting Hat more negatively puts it), not evil. There are several examples of Slytherins who are not evil (Merlin, Horace Slughorn, Severus Snape, Andromeda Tonks, Regulus Black) and we have at least two examples of evil non-Slytherins (Peter Pettigrew and Quirinus Quirrell--Mundungus Fletcher is a dodgy character as well, and he gives no evidence of being a Slytherin either). We have to remember that the statement "all wizards that have gone bad are Slytherins" was made by a very biased source--Hagrid, who had been expelled from school because a Slytherin used him as a scapegoat. This was before anyone knew about Peter Pettigrew, but since they wrongly believed Sirius Black had joined Voldemort instead of Pettigrew, they should have recognized that Dark wizards have emerged from other Houses.

Slytherin House has produced a larger number of Dark wizards than others for several reasons. First, according to Dumbledore, Salazar Slytherin (like Godric Gryffindor) valued students with a "certain disregard for the rules." In Gryffindors, a disregard for rules tends to mean a disregard for order (a trait which irritates Severus Snape to no end). In Slytherin, that disregard tends to mean that Slytherin students form their own moral code, which will be closely related to their personal goals--regardless of whether society at large agrees with their goals or values. Slytherins can therefore adopt moral codes that will eventually be considered "ahead" of their time (think Merlin, who mentored a Muggle king) or seem "behind" their time (think Voldemort). If the goals of the Slytherin students in question are admirable, then their moral code is likely to be admirable as well. But if their goals are evil, then their moral code will be no better. As a result, Slytherins such as Voldemort, who primarily want political power, are often attracted to the Dark Arts as a way of reaching their aims. (Other Slytherins may study the Dark Arts simply because they are curious, but Ravenclaws have been enticed by the Dark Arts for that reason as well.)

Second, Slytherin has historically been associated with purebloods, many of whom were attracted to Voldemort's pureblood supremacy ideals. Not all Slytherins are purebloods, however. Many are half-bloods, including Severus Snape and Voldemort himself. Harry Potter even speculates that the Slytherin Quidditch captain, Marcus Flint, looks like he is part troll--an interesting idea, especially considering ex-Slytherin Dolores Umbridge's negative reaction to anything she perceives as part-human. The Slytherin welcome letter in Pottermore leaves whether Muggle-borns may have been in Slytherin House during Harry Potter's time an open question.

Third, Voldemort was a Slytherin, and he naturally recruited from his own House. As a result, many of the students in Slytherin had family ties to the Death Eaters--especially those in Harry's year. Draco Malfoy, Vincent Crabbe, and Gregory Goyle all have Death Eater fathers, and Malfoy in particular was able to use his family's money to buy influence among his peers. Slytherin students older than Malfoy who attended Hogwarts during the calm between the two wizarding wars were likely not as influenced by the Death Eaters, however. Gemma Farley, the Slytherin prefect who authored the Pottermore welcome letter written for students during Harry's first year, even seems to think that the Dark and pureblood influences in her House have faded, reputation or no. Unfortunately, with Malfoy and his friends entering Slytherin, the balance of power gradually began to shift. Given the political climate and the presence of students like Malfoy, it should not be surprising that Slytherin House was perceived as being particularly friendly to the Dark Arts during Harry Potter's time at Hogwarts.

Still, J.K. Rowling has said that some of the older Slytherin students joined Slughorn's reinforcements in the second part of the Battle of Hogwarts (despite having been initially ordered out by McGonagall). Presumbly, those Slytherins, who would have been in Draco's year, had avoided conflict with Draco, his gang, and the Carrows while attending Hogwarts. They chose to fight when they thought it could make an actual difference--a reaction that combines the Slytherin strategic nature with the ultimate goal of stopping a rogue Slytherin from wrecking the entire country.