First off, Slytherin isn't evil. The answer I linked does a more than sufficient job of debunking that.
Similarly, that same answer shows that the "pureblood" stance is not part of Slytherin, as "nowadays you’ll find plenty of people in Slytherin house who have at least one Muggle parent."
There's nothing that says intelligence is the sole domain of Ravenclaw. There are quite a few examples of highly intelligent members of other houses, the most obvious being Hermione (Gryffindor), Dumbledore (Gryffindor), and ... Snape (Slytherin!).
Regarding ambition... Slughorn's "Slug Club" really was a form of ambition. While vanity was certainly a part of it, the club was Slughorn's "cash cow" (or Wizarding equivalent). He selected people based upon their influence rather than fame. Luna Lovegood, for example, was the daughter of the publisher of a very well known newspaper, the Quibbler. However, that wasn't sufficient to interest Slughorn, as there is no evidence that Luna was ever invited to join the Slug Club.
Rather, he choose people who could either directly provide favors, or create opportunities for him. This is precisely the type of ambition that Slytherin House was known for.
This wiki entry summarizes Slughorn's benefits from his efforts fairly well:
When helping others become famous, which, if successful, gained him influence and/or benefits from them, such as being able to recommend the next junior member of the Goblin Liaison Office, free Quidditch tickets or a box of his favourite crystallised pineapple. Horace preferring to be a "back-seat driver", obtaining things he desires by using his contacts makes his life comfortable cosy.