He is cunning and ambitious enough to come up with a way to get close to future achievers in order to get free stuff when they become big later in life. He seemed pretty good at picking those with talents. Therefore it made him good enough for Slytherin.
He isn't very brave or loyal so he wouldn't be a Hufflepuff or Gryffindor. He also doesn't seem to thirst for knowledge since he uses others to get where he is in the book so wouldn't be a Ravenclaw, hence the reason he is a Slytherin.
Not everybody in Slytherin was evil or a death eater - it just so happened that many of the characters who were death eaters came from that house. There were probably also a lot of unmentioned Slytherin's who had nothing to do with the Dark Arts. Severus Snape was a Slytherin and a Death Eater but was a spy for the Order of the Phoenix and eventually helped end the war.
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Horace Slughorn taught potions at Hogwarts while Severus was still a student. He later retired and only returned as a favour to Albus Dumbledore in what was supposed to be a one year deal. However, after Dumbledore's death he remained teaching Potions.
Professor Severus Snape starts out as the Potions master and Head of Slytherin House. Professor Minerva McGonagall is the Transfiguration teacher and Head of Gryffindor House.
Severus Snape
Professor Severus Snape. Dolores Umbridge also disliked him.
John Nettleship, Rowling's chemistry teacher and the character on whom Snape is based, taught at Wyedean school near Chepstow.
The wand of Severus Snape is of unknown length, wood and core materials.