We all know that Slytherin produced more dark wizards than any other house. We know that some extremely rude people *cough*Draco*cough* are Slytherins. But the idea that all Slytherins are evil is simply an overgeneralization.
Ambition, one of Slytherin’s defining traits, does not make a person completely good or completely evil. Ambition is simply having many goals that are difficult to achieve. Those goals can be good or evil. For example, Voldemort’s ambitious goal to kill Harry Potter is what makes him evil, not the fact that he’s a Slytherin. So, while most Slytherins (in history) have chosen evil ambition, some Slytherins have chosen good ambition, such as starting a business or refurbishing houses for wealthy couples.
In conclusion, it is up to the people in Slytherin to decide if they’re good or evil, not the fact that Slytherins are “supposed” to be evil. Because what use is it to trick millions of Harry Potter fans into thinking that there is an entire house that produces only evil people when in reality ced house can produce some extremely successful people?