Slytherin

Like Salazar Slytherin, its founder, Slytherin house values ambition, cunning, resourcefulness, and pure blood heritage. Most Slytherin students display a high level of Machiavellianism. The book also suggests that the hunger for power is a characteristic of Slytherins. The animal representing Slytherin is the serpent, and the house's colours are green and silver. According to Rowling, Slytherin corresponds roughly to the element of water.[4] The Slytherin dormitories and common room are reached through a bare stone wall in the dungeons. The Slytherin common room is a long, low, dungeon-style room, located under the Hogwarts Lake, furnished with green lamps, and carved armchairs.
Harry has developed an instant negative view of Slytherin and asks the Sorting Hat not to place him in that house because of its sinister reputation. Hagrid told him, "There isn't a witch or wizard who went bad who wasn't in Slytherin." This seems to be an exaggeration on Hagrid's part, as at the time, he believed that Sirius Black, a Gryffindor, was a follower of Voldemort (ironically the true perpetrator of Black's crimes was Peter Pettigrew, another Gryffindor). Still, Slytherin House seems to have produced more evil wizards than any other house, including Voldemort himself and almost all of his supporters. However, Slytherin itself is not evil; it's just that having ambition as a core quality results in a disproportionate amount of self-important, competitive students. There are some good Slytherins such as Slughorn, Phineas Nigellus Black, Andromeda Tonks, Regulus Black, and Snape (although his allegiances are unclear until the seventh book).
The Sorting Hat claims that blood purity is a factor in selecting Slytherins, although this is not mentioned until the fifth book. There is no reason to believe, however, that Muggle-born students are not sorted there, merely that pure-blooded students are more desirable to that house, as there are several examples of half-bloods in the house (Snape and Voldemort). In Harry Potter and the Deathly Hallows, a group of Snatchers claim that "not many Mudbloods" are sorted into Slytherin, which suggests that while Muggle-born Slytherins may be uncommon, they are not unknown.
When believing Harry to be dead and thinking that he has final victory in his grasp, Voldemort proclaims his intention to abolish the other three houses and force all Hogwarts students into Slytherin. This design is foiled by his defeat and death, after which Slytherin becomes more diluted in its blood purity, no longer the pureblood bastion it once was. Its dark reputation, however, does linger.[5]
A few notable Slytherins (for a complete list, see List of characters in the Harry Potter books):
• Lord Voldemort (as Tom Marvolo Riddle)
• Draco Malfoy (Also see The Malfoy Family)
• Bellatrix Lestrange
• Crabbe and Goyle
• Pansy Parkinson
• Severus Snape was the head of Slytherin house during the first six books.
• Horace Slughorn is the current head of Slytherin house.
• The Bloody Baron is the house ghost.