Why would DDM!Snape kill Dumbledore?

[Ceridwen]

No: HPFGUIDX 147596

ericoppen:
> >
> > I can come up with a scenario where Snape, _as Dumbledore's sworn 
> > man,_ would have to kill Dumbledore.
> > 
> > We don't know what the effects of that awful potion he swilled 
> down 
> > were.  For all we know, it was Essence of Concentrated Evil...

> Leah:

> I liked this idea, but can't tie it in to  DD's reaction to the 
> potion in the cave, which doesn't suggest to me that it will turn 
> him into the Big Bad.
> *(snip)*
> I see 
> this in Snape's mind as being the equivalent of the staking of Lucy 
> Westenra in 'Dracula'- a releasing of DD which also has the benefit 
> for Snape of releasing him from the vow, and allows him to get the 
> DEs out of Hogwarts. Grey Snape in fact.

Ceridwen:
I'll play.  Making a patchwork out of things I've read or thought 
myself after reading these over the past few months:

When DD destroyed the ring horcrux, he released the soul fragment, it 
immediately went for the nearest warm body, which was Dumbledore's.  
Between DD and Snape, they got the soul piece isolated in DD's hand.  
Since LV possession kills what he possesses much sooner than 
otherwise would have happened, the soul piece, being confined to a 
small portion of DD's body, and after whatever side effects there may 
have been from releasing the soul piece, quickly kills the skin of 
DD's hand.  But it is trapped in the hand and can go no further.

The potion in the cave is poisonous, and somewhat caustic.  Did it 
resemble anti-freeze to anybody else?  The poison in DD's system 
degrades the magic used to keep the soul piece in DD's hand as well 
as poisoning him.  So, we have DD becoming possessed.

Since DD is, obvious to me at least, more powerful than LV, and 
certainly more powerful than a bit of soul, he is able to stave it 
off while hoping to see Snape, to capture the bit before it hits the 
shoulder or somesuch.  Even with his being weakened by the physical 
effects of the poison.  But events on the tower preclude that from 
happening.  When Snape does arrive, DD is already losing the battle 
against the soul piece and its toxic possession.  And he can feel the 
invasion of LV's soul piece niggling at his brain.  He allows Snape 
to see this via Legilimency.  Here you get ericoppen's scenario 
coming into play, a BWA-HA-HA!Dumbledore in the offing and Snape 
killing him to prevent a full-blown 'I AM DARK LORD!  HEAR ME ROAR!' 
moment.

We do know that LV can possess, even as a memory.  We do know that 
something happened to DD's hand, which I think will be some sort of 
key to his mindset through HBP.  We know that thought images can be 
conveyed or plucked through Legilimency, and we know that both 
Dumbledore and Snape are practitioners of this art.  We do know that 
things work differently in the Potterverse because they have magic.

I don't think this will be the answer to the tower.  But it's as 
possible at this point as anything else.  And, it was a fun idea to 
play with.

Ceridwen.