“We’ve got a problem, Snape,” said the lumpy Amycus, whose eyes and wand were fixed alike upon Dumbledore, “the boy doesn’t seem able —”
But somebody else had spoken Snape’s name, quite softly.
“Severus . . .”
The sound frightened Harry beyond anything he had experienced all evening. For the first time, Dumbledore was pleading.
Snape said nothing, but walked forward and pushed Malfoy roughly out of the way. The three Death Eaters fell back without a word. Even the werewolf seemed cowed.
Snape gazed for a moment at Dumbledore, and there was revulsion and hatred in the harsh lines of his face.
“Severus . . . please . . .”
Snape raised his wand and pointed it directly at Dumbledore.
“Avada Kedavra!“

Harry Potter and the Half-Blood Prince – J.K. Rowling