Why did Snape call Lily a 'Mudblood'?

[potioncat]

No: HPFGUIDX 177613

 
> Jen: Guess I didn't make that part clear. ;)  What I meant was 
> something like: "Did learning more about Snape's and Lily's history 
> together help you go back and understand Snape's worst memory in a 
> new light?  Did the worst memory still work well in the story knowing 
> more information?"

Potioncat:
Again, from a half-baked position: In one brief flash of thought, I 
wondered if "all" the memories Snape removed prior to Occlumency 
lessons had to do with his relationship with Lily and with his vow to 
protect Lily's son. Are some of the ones he will give Harry at his 
death the same ones he removed here? 

Diverging a lttle, let's go to Slughorn's Christmas party. I wonder if 
Snape was concerned that Slughorn might say too much as he gushed on 
about Potions and Harry, Potions and Lily, Potions and Snape...