Not everyone in Slytherin ends up being evil. Examples include Snape, Regulus Black and Slughorn. In the same vein, not everyone who is evil was from Slytherin. Examples there are Peter Pettegrew (Gryffindor) and Quirrell (Ravenclaw) Slytherin is totally misunderstood. Slytherin's are just very ambitious, and they value self-preservation. They are less likely to follow their moral compass when making decisions, which is why there is a higher concentration of dark wizards in Slytherin. It's not like being put into Slytherin makes you an evil person.
