I'm assuming by "bad" they mean "dark." And, yeah, it's probably a stereotypical statement, except that, you're right, no one in the other houses seems to have any kind of inclination to go "bad." And I've never really thought about Quirrell or Lockhart. But if you look at it this way... Riddle seemed to be a bad egg from the beginning, based on Dumbledore's memories. The only difference was he seemed to act really good in school but was hiding his asshole-ness, while the current Slytherins were also jerks from childhood who just don't bother to hide it. Quirrell admits in the last chapter of Philosopher's Stone that he was very different before he met Voldemort that he was "all concerned about right and wrong". That sounds different from your typical Slytherin who seem to be rotten to begin with. And Lockhart, well, I never really considered him evil to begin with. The book really plays him off as an incompetent bumbling idiot more than genuinely malicious. But then again, he was competent and malicious enough to do all the memory charms we didn't actually see, so I guess that counts for something.

And I do hope that in the 7th book, they do something with the Sorting Hat's words. And I hope they give the Slytherins some reason, some common decency that will allow the other houses to want to band together. Maybe you're right. Maybe it is Snape's influence. I'm not entirely sure if I agree with it, though, since their upbringing seems to be an influential factor. Take Lucius and Narcissa Malfoy, for example.