One of the strongest similarities between both works would be how the protagonists in each seek to take an active step to integrate themselves into their worlds of being and find themselves to be rebuffed in the attempt to do so.  David's desire to purchase a gun is a desire to assert his own notion of masculinity.  It is his own desire to  fully integrate himself into his world that ends up causing him to actually leave it, recognizing that he will never find full acceptance or immersion.  In this, Fred Daniels operates in the same light.  His reemergence into society to show the police officers the truth is rebuked with them killing him.  Daniels demonstrates a new understanding about the world and his place in it, similar to David.  Yet, he is killed by the police who see him as nothing more than something that can be eliminated.  Just as David leaves into an unknown world, his presence not shown to be missed, Daniels is shown to be airbrushed in the same context.  Both characters seek to emerge out of obscurity to make a statement about their own conditions of being in the world, only to find that such action is futile in a condition that seeks to silence voice and not validate it.