Slytherin Defense Club

Sorting and Such

"There's not a single witch or wizard who went bad who wasn't in Slytherin."
—Wizarding prejudice against Slytherin House[src]

A serpent, symbol of Slytherin House
It is said that all Dark Wizards were in Slytherin (though this is untrue, as Peter Pettigrew was a Gryffindor and Quirinus Quirrell was a Ravenclaw), though how many were from this house before Tom Marvolo Riddle began recruiting Death Eaters there is unstated. As Slytherin was his own House, it was certainly most easy for him to recruit there.

It might also be possible (though quite unlikely) that the Sorting Hat simply sorts anyone with ill intentions into Slytherin, regardless of whether they possess cunning and ambition. Vincent Crabbe and Gregory Goyle were both incredibly stupid and did not have clever or cunning minds, but were still sorted into Slytherin. Though it does appear that perhaps they had more of the minor Slytherin traits since the cowardly Peter Pettigrew was obviously not brought into Gryffindor by bravery, but by chivalry instead. It is also likely that Crabbe and Goyle were simply placed there by process of elimination, since they exhibited virtually none of the qualities valued by any of the other houses (lacking the work ethic or honesty of Hufflepuff, the academic prowess of Ravenclaw, or the bravery of Gryffindor). Or the sorting hat simply took into account what house they wished to be placed in, which with their family history was most likely Slytherin.

However, it must be noted that Slytherins who oppose use of the Dark Arts and discrimination based on blood purity do exist, such as Horace Slughorn and Andromeda Tonks; Tonks was related to many Dark wizards and witches (most notably her sister Bellatrix Lestrange, her brother-in-law Lucius Malfoy, and her nephew Draco Malfoy), but chose to forsake the loyalty to her family in marrying the Muggle-born Ted Tonks. Also, the allegiance of Severus Snape was found to be ultimately towards Albus Dumbledore, serving as a triple agent for the Order of the Phoenix. Furthermore, there have been Dark wizards from other houses; Peter Pettigrew, who betrayed James and Lily Potter to Lord Voldemort, was a Gryffindor.

House ghost, The Bloody Baron
Cunning and ambition are easily turned towards ill purposes, but are not inherently negative; furthermore, daring, intelligence, and loyalty can as easily become negative. Slytherins may be thought less of by people because many of them seem to value their own lives more than the lives of others. When Harry was trying to run away, because he was afraid that his presence was putting his friends in danger, Phineas Nigellus saw him and said, "We Slytherins are brave, yes, but not stupid. For instance, when given the choice, we will always choose to save our own necks". There are exceptions to this rule, however, such as Snape, who Harry said was a "Slytherin and... probably the bravest man I ever knew", and Regulus Black, who gave his life in an attempt to destroy one of the Horcruxes of Lord Voldemort.

The truth is that while most Dark wizards are Slytherins, many members of Slytherin House do not have Dark leanings, and smaller groups of Voldemort 's supporters are aligned with other Houses. During the Battle of Hogwarts, while all of the Slytherin students left the school before the fighting started, several of them did return with Professor Slughorn as reinforcements to join the fight against Lord Voldemort and his Death Eaters.