"there never was a wizard who went bad who wasn't in Slytherin"

[Cassandra Claire]

No: HPFGUIDX 3805

I've just been rereading SS and came upon the passage where Hagrid 
says that "There never was a single witch or wizard who went bad who 
wasn't in Slytherin." Now, we assume he's speaking hyperbolically, 
don't we? Because at the time Hagrid is speaking, he would consider 
Sirius as a wizard who went bad, wouldn't he? And would that mean 
Sirius was in *Slytherin*? That seems impossible, so I'd have to say 
that Hagrid's statement is to be discounted...