ok this is my thoughts the entire book is based around the fact that you can cast no verbal spells and snape is thebest at it.... well think about it if snape said avada cadavera but thought a different spell perhaps one that shows alll the signs off death but without death, then maby hes not dead..... also the picture of dumbledore in his office is still asleep the say that a picture will only wake up when some one dies..... second harry is covered no one can see him so how did any freeze him on the towere maby i was dumble dore (non verbal) Maby him and snape had a plan to get snape inside of the death eaters "gang" so he could get inside info..... many thats why in the order of pheoniques harry was not aloud to listen to snapes conversations because voldemort can maby read harrys thoughts.... also not one time has the killing curse ever sent some one flying backwards it always just killed them.... well it flung dumby off the tower........ alsothis is off topic but...... the innnitials in the lockit are sirriuses brothers