When did Slytherin become the Evil House ?

[Doriane]

No: HPFGUIDX 88672


Hi all !

Many weird thoughts have been crossing my mind lately. I'll write 
down a few of them, and you can remind me how dumb and twisted I am :-
)

By the time Harry and Co enter Hogwarts, Slytherin has acquired a 
reputation as a House that prefers purebloods and that spawns evil 
wizards by the dozen. But I'm wondering when that reputation was 
born ? Has it always been that way, or did it become that way when it 
became obvious that many of LV's supporters were former Slytherin 
students ?

One important consequence, for example, would be that the Marauders 
could have been in Slytherin without it meaning anything. LV started 
gathering his supporters at about the same time the boys entered 
school, so they could have been sorted in Slytherin without it 
meaning that they were somehow evil, or that their families had long 
delved in the Dark Arts.

So can anyone point me to any canon that would indicate when 
Slytherin House's reputation was built ?

Del