When Alice arrives, she claims that she has evidence that Renesmee will never be a threat, and offers her hand to Aro to show him this. She then pulls away, saying "whatever I show you, you'll never change your mind!". The fight breaks out. Several people are killed, including Carlisle, Jasper, Seth, Leah, Jane and all three of Volturi leaders including Aro as the last.
As he dies, it's then revealed that all of this - the entire fight - is a vision of Alice's which is what she's showing him. He wants to save himself so he listens to the rest of her evidence (as in Nahuel) and the Volturi walk away as happens in the book.