J.k. Rowling said this quote

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 Dumble