Just watched Harry Potter and the Sorceror's Stone again, and wow...

[Lazy Kitty]

But it's kinda unfair how Slytherin are judged. In the latest movie when they take back the school McGonagall just orders all Slytherin to be thrown to the dungeons. Were they all evil douchebags?

Well, that's one of the important qualities you need to have to get in Slytherin.

You have to be willing to do ANYTHING to achieve your goals.
Well, that or be able to talk to snakes.

Even still, that seems rather prejudiced. Also since the houses are like classes, don't they have to be about as big? So how could the students be divided into houses just based on their qualities. If there are no enough complete douchebags to fill the Slytherin house, wouldn't that mean some of the less douchy people who'd otherwise end up some other house would just be dumbed into that house?

That's can't happen because the sorting hat doesn't know from before he gets put on what they're like.
There will always be enough douchebags , since their parents, who were also douchebags, raise their kids to be douchebags, which will do the same when it's their turn.
They're just a bunch of families who've been producing douchebags since the time of Salazar Slytherin and possibly before that.

A notable exception is Sirius Black, who didn't turn out to be Slytherin material like the rest of his family.