Okay, six.
Point is, that we should all realize Harry Potter grew with its fanbase. It was very whimsical and had a totally different tone for the first three, maybe four books. In Book 1 and 2, it was suited for children of Harry's age at that time (11-12). By the later books it has grown more complex and mature, just like its readers.
In the first few books, Slytherin could be the House of Evil because the setting was simply that....I don't want to say childish... It was simple there ya go. Draco Malfoy was not intended to be anything but a mustache-twirling jackass and neither was the rest of them.