you see what you expect to see

My word, Severus, that I shall never reveal the best of you?’ Dumbledore sighed, looking down into Snape’s ferocious, anguished face. ‘If you insist …

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

But this is touching, Severus,” said Dumbledore seriously. “Have you grown to care for the boy, after all?”
“For him?” shouted Snape. “Expecto Patronum!”
From the tip of his wand burst the silver doe. She landed on the office floor, bounded once across the office, and soared out of the window. Dumbledore watched her fly away, and as her silvery glow faded he turned back to Snape, and his eyes were full of tears.
“After all this time?”
“Always,” said Snape.

Course Dumbledore trusts you,” growled Moody. “He’s a trusting man, isn’t he? Believes in second chances. But me — I say there are spots that don’t come off, Snape. Spots that never come off, d’you know what I mean?

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