Davids sudden change from self-declared pacifist into somebody who wants to shoot Shauns mother was a volte-face of epic proportions, and completely planned by Wright as a reference to Sam Peckinpahs chiller masterpiece Straw Dogs.
David is modelled after Dustin Hoffmans character who is also called David, albeit David Sumner in both behaviour and to a certain extent looks. Wrights David has a similar moment to Peckinpahs David, where this once-meek man descends into violence. According to Wright, that moment comes when David is handed the rifle.