Why Didn't DD or Snape kill Voldemort?

[gelite67]

No: HPFGUIDX 152500

We know that DD didn't try to kill Voldemort at the MOM in the OOP.  
When Voldemort taunts him about that, DD cryptically responds that 
there are worse things than death. While I'm sure DD heartily believes 
that is true, I don't believe that is the real reason he didn't kill LV.

We also know that Snape has come face to face with LV since LV's return 
to corporeal form.  So, if Snape is really DD's man, why didn't he kill 
LV then?  Sure, the DEs probably would have finished Snape off at that 
point, but if Snape truly is a member of the Order, wouldn't he be 
prepared to make that sacrifice? (Of course, I am assuming that LV 
allows his DEs to be "armed" with their wands in his presence, in case 
he needs them to defend him.)

The only reason I can come up with is that DD fears that Harry will die 
if anyone but Harry kills LV.  Perhaps DD believes this because of the 
prophesy or the connection forged between Harry and LV by the scar, or 
both?

Any other ideas?

Angie