Rowling interview transcript

[Zara]

No: HPFGUIDX 180323

> Irene:
> I just don't get it, how it sits together for her - Slytherins are 
not 
> all bad, and yet someone who becomes a better person, stops being 
> suitable for Slytherin.

zgirnius:
I think the source of the confusion is the purpose the house serves 
in the book. 

On the one hand, I believe in Rowling's imagination of the 
Potterverse, it is merely one of four houses in Hogwarts, in which 
roughly 1/4 of the wizard population of the British Isles is/was/will 
be educated. Thus, it is far from true that they are all bad, or that 
there is no one good ever in the house, despite some examples of very 
bad apples coming fron that house.

It is also the house from which the major villain of her story comes, 
and also young Harry's greatest antagonists in the school setting, 
his rival Draco and the teacher he hates, Snape. Since the story is 
centered about Harry, this means most of the Slytherins we meet are 
in Voldemort's gang or in Draco's, but see above, the quarter of the 
merchants in Hogsmeade, or Healers at St. Mungo's, or what have you, 
who are Slytherins, are invisible (and irrelevant) to the readers.

As a result, there is what I consider to be a fan misconception, that 
Slytherin is the 'bad house'. I don't think Rowling envisions it so, 
I think the impression was created naturally out of the function in 
the plot/story for the house. And if a fan has this assumption in 
his/her mind when asking a question, what is Rowling supposed to do, 
other than repeat that Slytherins are not all bad?

Based on her answer, she apparently thinks Snape proved himself, as 
an adult, a better fit for Gryffindor. The canon quality which he is 
said (by no less an authority than Adult!Harry) to exemplify above 
all other characters, is courage, not moral rectitude. And courage 
happens to be *the* Gryffindor trait.