Snape didn't have to

[caesian]

No: HPFGUIDX 135796

The fine print of the Unbreakable Vow argues against the theory that  
Snape is simply ESE.

Snape agreed to three conditions in his unbreakable vow:

"Will you, Severus, watch over my son, Draco, as he attempts to  
fulfill the Dark Lord's wishes?"
"And will you, to the best of your ability, protect him from harm?"
"And, should it prove necessary ... if it seems Draco will fail ...  
will you carry out the deed that the Dark Lord has ordered Draco to  
perform?"

I don't know exactly how the Unbreakable Vow works, or how the  
precise wording and/or intentions of the participants is assessed -  
but, it looks to me as if, like Prophesy, a certain amount of human  
manipulation of circumstances is possible.

If these things are true, then Snape cannot *simply* be ESE.  If  
Snape is ESE, and cares only for himself, killing DD would not have  
been as attractive as simply *strongly encouraging* Draco to kill  
him.  Draco was present, and nothing in the wording of the Vow  
prevents this.  He has already repeatedly offered Draco help, so this  
is not forbidden.  Should Draco have done the dead with Snape's  
encouragement, the end, for the vow, would have been similar.   
Further, he was sworn to watch over Draco, and protect him from harm  
(and no doubt Draco will be harmed for his failure regardless of  
whether DD is dead).

IMO, SS did not HAVE to kill Dumbledore himself under the terms of  
the vow.  The fact that he did so, rather than attempting to make  
Draco do so, is significant.

Cheers,
caesian