Haven't seen the movie yet, but Snape sacrifices himself in the book to convince Voldemort that by killing him he's removing the last obstacle to the power of the elder wand, and will thus be invincible. That's why Voldemort no longer felt it necessary to protect the Nagini-horcrux at that point, his crucial mistake. Snape gave up his life (instead for instance of half-lying to him that he didn't disarm Dumbledore but doesn't know who did etc.) so Voldermort would throw caution to the wind, thus buying perhaps the only chance for Harry to destroy the then overprotected Nagini and face a Voldemort unaware he isn't invincible.