Well, you must also remember that in the case of the Basilisk (and some other Dark Creatures) they were bred by Dark Wizards to be what they are. Their entire existance is due to the Dark Arts. Now, that said, there are two ways to look at this information:

1 They had no control of their own developement and therefore no choice in their own nature, meaning they are not truely Dark since they made no choice to be so. They merely act on those instincts they are born with and have no choice but to follow.

2. They were created by Dark Wizards to be Dark creatures adn although they had no choice in the matter they cannot be other than Dark since their entire existance depends on it. A Basilisk is not a Basilisk if it did not kill with a glance, have a potente venom and hunt prey. Sure, it's no tth ecreature's fualt, but that doesn't make it any less Dark and dangerous, or by make up alone they can never be nought but Dark Creatures.

As for S. Slytherine - this may make me unpopular, but IMO he may very well have descended into the Dark Arts himself. Looking strictly at circumstantial evidence (since it's all we have) his relations to peers fractured and broke, he became moody and mean, created teh chamber and bred a Basilisk ot dwell there. He never (that we know) sought the elimination of Mudbloods, but he definitely wanted their exclusion at Hogwarts and seemed willing to go to great lengths to achieve it. Though I doubt he was always a Dark Wizard, I do believe it is possible that he began that descent while leaving Hogwarts and either recovered or not after having left, since we have no further accounts of him after his life at Hogwarts.


-K-