As for the Slytherins who sided with Voldemort, I kind of feel like a lot of them did so just to be accepted. Slytherin was hated and ostracized by the other houses already, so what did they have to lose? That's the way I see it. And anyway, not all of them sided with him. So they're definitely not all bad.
Besides, Snape was a Slytherin. And he was good, even if no one knew it.