In the End
Disclaimer: I don't own Harry Potter, J.K. Rowling does, and also DH spoilers here...
Severus Snape knows he failed the boy. It's funny how being loyal to his master his last few hours of his life, doesn't pay for his redemption, nor does being in love all these years with Lily Evans doesn't matter. In the end, he knows that even though he failed the boy that somehow Harry, no Potter, will end the way. He just hopes that Harry can find away without dying.
In the end, Severus Snape knows that Harry Potter will win the war. He's at peace, he can feel his redemption coming forth.
And then he feels nothing at all.