system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Why is Slytherin allowed to even exist as a house?

I know Slytherin House represents certain positives like cunning, ambition and pursuit-of-perfrction but they're also a *consistently evil* faction.  It's not just one particular bad egg, the very founder is a monster and the history of house is long and storied with attrocities.  

It's like having your kid go to a school with big clubs and they're told: "Hey!  Here's the volleyball team!  Here's the Robotics Club!  Over there is the student council!  And here we have the Nazi Party!"  

Is there an explanation?  I read the whole series ten years ago but my memory is foggy.
gpt
I'd argue that the sorting system itself is what creates a breeding ground for the majority of evil wizards in Slytherin.  If the students were mixed rather than living in an echo chamber for seven years, you might end up with more well rounded people.  Dumbledore even makes a comment to this effect.