Max tries to distract Ari while she tells the others to run away. She asks him what happened to him, and why he became an Eraser—which upsets him.
He obviously has some issues with Jeb (his father) and feels like he's the one who was left behind, while Max was considered the perfect hybrid. The whole thing almost makes Max feel sorry for him, emphasis on the almost.
Max kicks Ari when he's not paying attention, and he slips on the tunnel ledge to fall on his back. She screams at Fang to take the rest of the flock and run while Ari pops up and punches her.
As he pummels her, he loses his balance again and she kicks him in the throat. He goes down hard and hits his neck against the side of the tunnel.
Max realizes with shock that she's broken his neck—and that he's dead.