He knows. The thought came to Harry, and he couldn't have said in words just what the Potions Master now knew; except that it was clear that Severus knew it.

The Dark Lord spoke the words "Hyakuju montauk" without pausing in his stride, accompanied by a jab of his wand; and Severus staggered before he lifelessly drew himself up beside the door once more.

"What -" Harry said, as he followed. "What did you -"

"Just fulfilling my obligation to my faithful servant. It shall not kill him, as I promised you." The Dark Lord laughed again.

I don't understand either of these. Though this in the mix and I still don't understand.

I went to the Dark Lord intending to sell him the prophecy in exchange for Lily's love becoming mine, by whatever darkness was required to achieve it.

Severus shook his head. "Too many students would remember me as the evil Potions Master. No, Minerva. I will go someplace new, and take a new name, and find someone new to love."

I get the character arc "Snape's obsession was used by Dumbledore and Voldemort, but he has finally gotten over Lily and can move on" -- it's just these specifics I don't understand.