В книге не написана сцена того, как Снейп рассказывает Волду о пророчестве. Там могут быть разные варианты.

“What request could a Death Eater make of me?”
“The— the prophecy . . . the prediction . . . Trelawney . . . ”
“Ah, yes,” said Dumbledore. “How much did you relay to Lord Voldemort?”
“Everything—everything I heard!” said Snape. “That is why—it is for that reason—he thinks it means Lily Evans!”
“The prophecy did not refer to a woman,” said Dumbledore. “It spoke of a boy born at the end of July— ”
“You know what I mean! He thinks it means her son, he is going to hunt her down—kill them all— ”
“If she means so much to you,” said Dumbledore, “surely Lord Voldemort will spare her? Could you not ask for mercy for the mother, in exchange for the son?”
“I have— I have asked him— ”
“You disgust me,” said Dumbledore, and Harry had never heard so much contempt in his voice. Snape seemed to shrink a little, “You do not care, then, about the deaths of her husband and child?
They can die, as long as you have what you want?”

И то что Волд побежит убивать ребёнка, Снейп об этом ЗАРАНЕЕ знать не мог.