attackfish

He has read the complete prophesy, and he did what he did with the full knowledge that he was making Harry the other object of the prophesy. After killing Neville as a baby and then finding out that Harry also fulfilled the terms, Voldy, who has never been terribly good at the logic, figures that fate will conspire to keep shoving people at him who fulfill the prophesy's terms until he picks one. He picked Harry, "marked him as his equal," and immediately made him a Horcrux. Voldy marked him as the only person able to defeat him, and simultaneously made it impossible for Harry to defeat him. He didn't take into account that he could be defeated without being killed, say, or that the Horcrux inside Harry could be destroyed without destroying Harry, because as I said, Voldy's not up on the logic thing.