Dumbledore: Ah, Professor Snape, let's see... what are your qualifications?

Snape: I knew more dark spells before I entered Hogwarts than most fully grown wizards. I have seen first hand what the Death Eaters are capable of. I risk my life for the student body every year by defending them against the Dark Arts. I am a skilled Occlumens. I got top marks in every single class I took at Hogwarts. I passed all my O.W.L.s and N.E.W.T.s. with flying colours. I am a spy for the Order and a good guy. Even the worthless moronic dunderheads seem to pass my classes. I don't have Voldemort attached to my head. Nor am I a fraud. Or disguised using Polyjuice Potion -

Dumbledore: Yes, that's all good and well. Are you interested in killing Harry Potter?

Snape: Of course not.

Dumbledore: I'm sorry, Severus, I'm going to have to decline your application.