Power hungry Slytherin
loved those of great ambition.
(This is from the sorting hat's song in Goblet of Fire)
In Slytherin you'll make your real friends,
Those cunning folk use any means
To achieve their ends.
(Also, from the sorting hat's song in The Sorceror's Stone
There is no good and evil, there is only power and those too weak to seek it.
(Professor Quirrel to Harry in the Sorceror's Stone)