Quote from : Harry Potter and the Half-Blood Prince Book

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.