A couple of little theories!

[zanooda2]

No: HPFGUIDX 162258

> Jenni from Alabama wrote:

<SNIP>

> It's not just that Snape actually killed Dumbledore, it's the look 
> on his face that JKR describes just before he did it - of revulsion 
> and hatred. It's the comments he makes to Harry about Lily being 
> a 'Mudblood'. 

zanooda:

Well, Snape sure is nasty, mean and all the rest of it, but let us be 
fair: never ever did he make any comment to Harry about Lily being 
a "Mudblood". In fact, he never mentions Lily in Harry's presence, 
not even once. He never misses an opportunity to tell Harry how 
arrogant, cowardly etc. his father was, but Lily seems a tabu 
subject. It's quite a contrast, actually, and this is one of the 
reasons why many people think that Snape loved Lily or at least 
respected her as a friend.

As for "revulsion and hatred", it's possible that these feelings are 
not intended for DD, but for Snape himself, that's how Harry felt 
when he had to force-feed DD that green potion in the cave, it's 
described in practically the same words in both cases. Or maybe Snape 
hated DD at that moment for making him do what had to be done. 

Of course, maybe I'm not very objective here, because I'm absolutely 
sure that Snape is DDM, otherwise I don't see much sense in what 
happened so far.