Voldemort did not want to lose the elder wand to the churning black waters, not when he had come so far, to  see  the wand within his grasp. No, he must kill the girl, and then it would be his. Then Severus. And then, finally, Harry Potter—he could end it all in one moment, one fell swoop. It was like a tunnelling prisoner seeing daylight for the first time. Whatever he had to sacrifice in this moment, he would. Unmindful of his struggling, drowning soldiers, he landed on the grounds of Hogwarts and raised his wand towards the girl. 