TL: DR : Ele tinha certeza de que ele tinha 100% garantido de matar Harry. Ele, portanto, não precisava fugir. Ele também não sabia que todas as outras Horcruxes foram destruídas antes que Harry lhe dissesse isso.

Isto é elaborado nos livros (note que nos livros, Nagini foi morto muito antes da última luta - fora do castelo)

“Potter doesn’t mean that,” he said, his red eyes wide. “That isn’t how he works, is it? Who are you going to use as a shield today, Potter?”

"Nobody,” said Harry simply. “There are no more Horcruxes. It’s just you and me. Neither can live while the other survives, and one of us is about to leave for good. . . .”

“One of us?” jeered Voldemort, and his whole body was taunt and his red eyes stared, a snake that was about to strike. “You think it will be you, do you, the boy who has survived by accident, and because Dumbledore was pulling the strings?”

_{(DH, Ch. 36, "The Flaw in the Plan")}

E mais abaixo:

“Is it love again?” said Voldemort, his snake’s face jeering. “Dumbledore’s favorite solution, love, which he claimed conquered death, though love did not stop him falling from the tower and breaking like an old waxwork? Love, which did not prevent me stamping out your Mudblood mother like a cockroach, Potter— and nobody seems to love you enough to run forward this time and take my curse. So what will stop you from dying now when I strike?”

“Just one thing,” said Harry, and still they circled each other, wrapped in each other, held apart by nothing but the last secret.

“If it is not love that will save you this time,” said Voldemort, “you must believe that you have magic that I do not, or else a weapon more powerful than mine?”

“I believe both,” said Harry, and he saw shock flit across the snakelike face, though it was instantly dispelled; Voldemort began to laugh, and the sound was more frightening than his screams; humorless and insane, it echoed around the silent Hall.

“You think you know more magic than I do?” he said. “Than I, than Lord Voldemort, who has performed magic that Dumbledore himself never dreamed of?”

_{(DH, Ch 36.)}

Porque ele achava que ele era o mestre da Varinha Ancestral

Voldemort estava operando sob a impressão de que ele era o mestre da Varinha dos Anciões. A varinha dos Anciões é a varinha mais poderosa de todas, então, simplesmente, Voldemort achava que ele tinha garantida a vitória sobre Harry. É provavelmente por isso que ele não tentou escapar - uma vez que Harry Potter foi morto, ele poderia fazer mais horcruxes para si mesmo mais tarde. Ele foi confrontado com a chance de matar Harry, uma oportunidade que pode não se apresentar no futuro. Dado o ódio de Voldemort por Harry, e acreditando que ele era o mestre da Varinha Ancestral, ele não viu nenhuma necessidade de escapar.