Why did Snape call Lily a 'Mudblood'?

[potioncat]

No: HPFGUIDX 177644

Pippin wrote: 
> I think it's significant that Hagrid and Slughorn are the two
> characters who find it hard to believe that Snape could have
> gone back to Voldemort's side. 


Potioncat: 
Yes, and I would very much like to see how they related to Headmaster 
Snape. I have the feeling from Minerva's comments, that Slughorn was 
not unfriendly toward him.