But somebody else had spoken Snape’s name, quite....

“ 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 gazed for a moment at Dumbledore, and there was revulsion and hatred etched in the harsh lines of his face.“Severus . . . please . . .”Snape raised his wand and pointed it directly at Dumbledore. ”A jet of green light shot from the end of Snape’s wand and hit Dumbledore squarely in the chest. Harry’s scream of horror never left him; silent and unmoving, he was forced to watch as Dumbledore was blasted into the air. For a split second, he seemed to hang suspended beneath the shining skull, and then he fell slowly backward, like a great rag doll, over the battlements and out of sight. ”

J.K. Rowling

From : Harry Potter and the Half-Blood Prince