It is very important to consider the case of Draco Malfoy when speaking of Slytherin House. He is a revealed as a character with depth in the 6th book, HBP, which is essential to recognize.

J.K. Rowling may have portrayed the Slytherins as terribly mean because she wanted to emphasize that point. It was important for readers to understand that Slytherins were typically darker than the other houses in order for them to understand the origins of Lord Voldemort, the Death Eaters, etc.