Slytherin corresponds roughly with the element of water.
Traits and ValuesEdit
Slytherins tend to be ambitious, shrewd, cunning, strong leaders, and achievement-oriented. They also have highly developed senses of self-preservation. This means that Slytherins tend to hesitate before acting, so as to weigh all possible outcomes before deciding exactly what should be done.
According to Albus Dumbledore, the qualities which Salazar valued in the students he chose included cleverness, resourcefulness, determination, and "a certain disregard for the rules." Dumbledore noted that all of these were qualities possessed by Harry Potter, who was in Gryffindor.
The Sorting Hat claims that blood purity, is a factor in selecting Slytherins. This suggests that it bases its decisions more upon the views of its members than on any accurate test of a student's lineage. Muggle-born Slytherins exist, but are very rare, as noted insultingly by Scabior the Snatcher. There have also been definite examples of half-bloods sorted into the house, including Tom Marvolo Riddle and Severus Snape. Harry Potter was nearly sorted into Slytherin, but ended up in Gryffindor instead. Also, pure-bloods are not automatically placed in Slytherin. There have been various examples of pure-bloods sorted into other houses, including the Weasley Family, Sirius Black, Neville Longbottom, and James Potter all being sorted into Gryffindor house, Ernie Macmillan being sorted into Hufflepuff, and Morag MacDougal and Stephen Cornfoot being sorted into Ravenclaw.
Contrary to the wishes of the other founders of Hogwarts School of Witchcraft and Wizardry, Salazar Slytherin wanted a stricter policy of admission to the school —one limited strictly to pure-blood students. The controversy this created apparently led to him leaving the school and the Chamber of Secrets behind. A millennium later, the house's students are still considered affiliated with Salazar's reputed views, and open expression of pro-blood purity sentiment does seem to be more common in Slytherin than in other Houses.
This disagreement over blood purity, which caused Salazar Slytherin's friendship with Godric Gryffindor to break, is the origin of the feud between Slytherin and Gryffindor, the two most competitive of the houses. Slytherin and Gryffindor are generally rivals in Quidditch and for the House Cup, which the former had won seven years in a row prior to Harry Potter's arrival at Hogwarts in 1991. It is also rare for Slytherin and Gryffindor students to be friendly with one another, and common for them to share animosity. This was particularly the case during the First and Second Wizarding Wars. During the latter conflict, only one known Slytherin student (Falin Snape) joined an association known as Dumbledore's Army; however, this could be explained by the fact that only those Hermione Granger, Ron Weasley, and Harry Potter trusted were invited. Some Slytherins declined to defend their school in the Battle of Hogwarts, with some joining the Death Eaters.
Despite the animosity between the two houses, there are exceptions, such as Slytherin Severus Snape and Gryffindor Lily Evans, who were best friends until Snape's use of the slur "Mudblood" and his aspirations to be a Death Eater caused Lily to sever their ties, as well as Slytherins Falin Snape and Gryffindors Hermione Granger, Harry Potter and Ron Weasley being friends from their fourth year on.
It is said that all Dark Wizards were in Slytherin (though it 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 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, cunning minds, but were still sorted into 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; the latter was related to many different Dark wizards and witches (most notably her sister Bellatrix Black, 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 Voldemort, was a Gryffindor.
Cunning and ambition are easily turned towards ill purposes, but are not inherently negative; furthermore, daring, intelligence, and loyalty can as easily become negative. A reason Slytherins may be thought less of by people is perhaps 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 Black 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 Severus 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 no Slytherin students returned to fight in the battle, other than Fegas Praygun and a few select others. Whether this is because they all believed in what Voldemort was doing, or because Minerva McGonagall ordered the entire house to leave due to a single Slytherin student suggesting the student body hand Harry Potter to Voldemort, is unknown; it is probably the latter, as none of the Slytherins present moved to defend Harry from the suggestion, as the other three houses did.