Being a Slytherin is... difficult sometimes; even though the prejudice from the wars had faded over the decades, there were still a lot of people who equated ambitious with evil.

But that's not what Slytherin is: Slytherin is the girl whose ambition is to cure the cancer that killed her mother and will stop at nothing to find that cure no matter who tells her it's impossible; Slytherin is the boy who wants to become the best singer the world has ever seen and will use every free second to make that happen (and maybe even borrow a few moments from unimportant things like sleep or meals or classes); Slytherin is the pair of twins who were ignored as children and want to make something of themselves; Slytherin is every child that ever had a dream and the determination to make it happen.

So that's what Slytherin is, because ambitious does not equal evil and everyone has a right to their dreams.