Continuity mistake: At 12 Grimmauld Place, when Harry and Sirius are in the room with the Black family tree, as Sirius says, "My parents with their pure-blood mania," the area of the wall with both Sirius's and Andromeda's names and burned off pictures is quite visible (as in other shots as well). Bellatrix's name is beside Andromeda's (her sister), but there is no picture over Bellatrix's name banner in this shot (only branches and leaves), nor in any other shot, except in the previous close-up, as Sirius says, "My deranged cousin," where there is a picture of Bellatrix over her name. (01:13:25)
Answer: When Voldemort originally attacked Harry, he was acting on information provided to him by Snape. But Snape's information was not complete, ergo why Voldemort lost his powers. He wants the prophecy in order to hear it in its entirety, because he believes it will give him the information he needs to kill Harry.
Cubs Fan ★