Your first step in thinking through your installation is deciding what sense of good and bad you wish to emphasize. The terms in English are ambiguous. We can use them to mean ethically virtuous or evil, but we can also use them to mean skilled or well crafted. If I describe someone as having a "good backhand" in tennis, I am referring to a skill rather than a moral characteristic; a "whitewater kayak" or "bad car" again have a sense of functionality rather than morality.
If you choose the moral theme, you might divide the space in half and think about "good" and "bad" ways of treating space -- one side covered with litter, cigarette butts, drug paraphernalia, graffiti, etc. and one side containing a garden. Another possibility would be to collect good and bad versions of various objects (a "good" vs "bad" backpack, pair of socks, pencil, etc)