Another reason for Dumbledore's plea to Snape

[meglet2]

No: HPFGUIDX 136292

Besides all the arguments about Dumbledore prearranging something 
with Snape for just such a crisis as occurred on the tower, I 
thought of one further reason which may not have been discussed. I 
am sure from reading and rereading the last chapters that the potion 
from the cave was killing DD. He says sarcastically that it was 'no 
health drink' and we see him getting progressively weaker throughout 
the scenes that follow the cave apart from the adrenalin rush that 
the dark mark above the tower produced. Despite his mental and 
spiritual calm in the conversation with Malfoy and the Death Eaters 
he is physically deteriorating fast. Several times JKR refers to him 
slipping further and further down the wall. He initally insists 
Harry fetch Snape because as such an expert potion maker Snape is 
likely to be the only one able to get an antidote. When it becomes 
evident that the time for all antidotes is past DD begs Snape to 
deliver the final blow, not so much as an act of mercy but to spare 
Harry the crippling guilt of having been the one to kill DD. 

Harry had to force DD to take the potion at DD's own prior 
insistence. (Note the words JKR uses of Harry's feelings, hatred and 
repulsion at the act, echo the hatred and revulsion etched on 
Snape's face as he launches his killing curse). If he later came to 
realise that it was drinking that potion that killed DD, even if he 
did it under orders, Harry would be DD's killer. He could not live 
with that and he could not have energy for the fight ahead with 
Voldemort. But seeing Snape, whom he hates and distrusts anyway, 
doing this deed, fires him as nothing else could and spares him the 
guilt of bringing about DD's death. Snape is also now in the best 
possible position to undermine Voldemort from within the 
organisation. IMO it is one more way in which we see Snape, albeit 
reluctantly and possibly even bitterly, acting as Harry's protector 
at DD's wishes. Perhaps!

Mercia