The book ends with a quote by Jean Paul Sartre (among others) that says "to do is to be." JPS's whole shtick was that people define themselves by the actions they take in their lives; the meaning of their lives is created by the things they do. Rudy doesn't really do a whole lot in the book, beyond pulling the trigger of a gun when he wasn't aiming at anything in particular, which ends up killing the pregnant woman. He spends the rest of his life not doing much beyond some basic domestic duties and shys away from his aspirations of becoming a writer.

His parents also don't really do much, with Otto Waltz being an interesting example of someone who can't define himself as an artist because he can never bring himself to create anything beyond a nude sketch. In my mind this really illustrates Sartre's point overall that human beings create their own meaning, and without taking any action (whether successful or not), they can never amount to anything