Just realised. OH man the details in these books.
Dumbledore told Snape of how harry had to die, even before he showed Harry the memory of Hokey. The man was a genius.
In chapter 17 of HBP, Hagrid tells harry and hermoine how dumbledore's angry at Snape.
“Hagrid, why is Dumbledore angry with Snape?” Harry asked loudly.
“Shhhh!” said Hagrid, looking both nervous and angry. “Don’ shout stuff like that, Harry, d’yeh wan’ me ter lose me job? Mind, I don’ suppose yeh’d care, would yeh, not now yeh’ve given up Care of Mag —”
Then Hagrid tells them -
“Well — I jus’ heard Snape sayin’ Dumbledore took too much fer granted an’ maybe he — Snape — didn’ wan’ ter do it anymore —”.
“Do what?”
“I dunno, Harry, it sounded like Snape was feelin’ a bit overworked, tha’s all — anyway, Dumbledore told him flat out he’d agreed ter do it an’ that was all there was to it. Pretty firm with him. An’ then he said summat abou’ Snape makin’ investigations in his House, in Slytherin.
Fast forward to DH, Prince's tale..
Dumbledore glanced around to make sure that they were alone. They were close by the Forbidden Forest now, but there was no sign of anyone near them.
“After you have killed me, Severus —”
“You refuse to tell me everything, yet you expect that small service of me!” snarled Snape, and real anger flared in the thin face now. “You take a great deal for granted, Dumbledore! Perhaps I have changed my mind!”
“You gave me your word, Severus. And while we are talking about services you owe me, I thought you agreed to keep a close eye on our young Slytherin friend?”
Snape looked angry, mutinous. Dumbledore sighed.
“Come to my office tonight, Severus, at eleven, and you shall not complain that I have no confidence in you. . . .”
Then Dumbledore tells snape of how harry had to die.
Mind you, this is the same night, where ron was cursed. This part of discussion happened even before Dumbledore showed harry all the memories. Even before, Harry got the memory from Slughorn.
Dumbledore was such a genius. SALUTE.