The narrative takes time to paint even Slytherins who dont fight for Lord Voldemort as selfish, self-serving and cowardly.This makes their house an easy target for feelings of ill-will from characters within the series and even from fans. Hence, the reputation that Slytherin has of being evil as a whole

There is a Quote for slytherin's for everyone in the hogwarts:

You'll find that many Slytherins have a great sense of family and loyalty.