Killed?" said Hagrid loudly, staring down at Harry. "Snape killed? What're yeh on abou', Harry?"
"Dumbledore," said Harry. "Snape killed... Dumbledore.

You think it - wise - to trust Hagrid with something as important as this?"
"I would trust Hagrid with my life," said Dumbledore.

Snape raised his wand and pointed it directly at Dumbledore.
"Avada Kedavra!

Draco, do it, or stand aside so one of us -" screeched the woman, but at that precise moment the door to the ramparts burst open once more and there stood Snape, his wand clutched in his hand as his black eyes swept the scene, from Dumbledore slumped against the wall, to the four Death Eaters, including the enraged werewolf, and Malfoy.
"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 forwards 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 etched in the harsh lines of his face.
"Severus ... please ..."
Snape raised his wand and pointed it directly at Dumbledore. "Avada Kedavra!

you see what you expect to see

Harry constantly repeated Dumbledore's final words to himself. "I will only truly have left this school when none here are loyal to me. ... Help will always be given at Hogwarts to those who ask for it. "But what good were these words? Who exactly were they supposed to ask for help, when everyone was just as confused and scared as they were?