Why did Snape call Lily a 'Mudblood'?

[Carol]

No: HPFGUIDX 177657

Carol earlier:
> 
> > I wish I had time to answer this post more fully. Let me just say
 that I disagree with it completely.
> 
> "Carol:" (really JP)
> 
> I am willing to concede to several of the points you make as there 
are some things that I hadn't taken into consideration.  I am going to
go back and re-read that chapter to see if my mind changes.  But
before I do... <snip the rest>

Carol:
Really me this time. Just posting here to note that the comments
apparently attributed to me are really responses by JP to my post.

Carol, appreciating JP's offer to reconsider on rereading but hoping
he'll forgive her for not answering the questions he raised, which
lead into the realm of speculation rather than analysis of the text