Welcome to Harry Potter Answers. What would you like to know?

Yes. It goes generally if you're evil, you're a Slytherin, but that doesn't mean if you're a Slytherin, you're evil. For example-Severus Snape, Horace Slughorn, Andromenda Tonks, Draco Malfoy, Malfoy family, and possibly Albus Severus (His initials were A.S.P. or asp, a type of snake).

There were probably plenty of good Slytherins during Harry's time at school, but because of the traditional animosity between gryffindors and slytherins, they weren't seen much in the books. It can be safely presumed that slytherin students not mentioned were good, simply because they were not associated with Malfoy's gang of blood status obsessed Voldemort supporters. During the Battle of Hogwarts, none stayed, because slytherins are naturally self serving, and its rare for them to risk themselves without some promise of gain or reward. Slughorn is an exception.