He knew. Adrenalin clawed at Harry’s chest. He   knew.  Snape was  lying. He might be an excellent occlumens, but whatever Dumbledore had told him about Harry’s condition had obviously shaken him, and Harry didn’t need to be able to read his mind to see that he was deceiving him. And for him to be so shaken, there was only one thing that Dumbledore could have told him. That Harry was a horcrux. That he had to die. In the memory that Snape had once given him, Dumbledore had stressed the necessity for Voldemort to be the one to administer the killing blow, and so Harry inferred that he’d known there was a chance that Harry would survive. The cursed wound on his neck though almost certainly ensured that his death would be permanent. It explained why Snape was abruptly pulling out all the stops to try and heal him - no doubt his love for Harry’s mother compelling him to try and save her son.