still not sure about snape though...
he performed the avada kedavra against dumbledore
without so much as a fight - and against dumbledore?
only probably the most gifted wizard of all time!
i know that he didn't have his wand,
but he still had his really powerful mind to block the curse!

his "please severus" really could have meant anything.

i am more inclined to believe
them two had an agreement that snape will perform
this curse in case this situati0n happened,
and that they had somehow managed to devise a
counter-curse for it.