Why would DDM!Snape kill Dumbledore?

[ericoppen]

No: HPFGUIDX 147589

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...and in 
the few seconds they had to confer, using Legilimency, Dumbledore said 
something to Snape like:  "Severus!  I'm turning evil! For God's sake, 
kill me quickly!  I can't control myself much longer!  I can 
practically feel a BWA-HA-HA coming on me as I'm lying here!"  But all 
that came out of his mouth was "Severus...please!"

The knowledge that, if I didn't, _Dumbledore would turn evil_ would 
have had me launching an A-K as fast as the words could escape my 
lips.  An evil Dumbledore would be a thousand times the menace that 
Tom "I-am-Lord-Voldemort-BWAHAHAHA" Riddle ever could have been, if 
only because _people trusted him!_  

--Eric, who maybe understands evil a _leetle_ too well...