Well i think that in the books there had to be the clear villain and the clear hero. and the hero and the villain had to come from somewhere. so slytherin and gryffindor where the places they had to come from.

I mean it does suck that JK didn't give the slytherins a chance to step up and fight or go the traditional route of a slytherin. i wish she would have given them a choice at least

she made so many references to things in life and how she tired to relate modern and age ole problems to the wizard world, and she really dropped the ball on this one