To develop the ideas a bit more fully.  For example, the notion of "fear" isn't really a theme yet (a theme is an idea you should be able to state in a short sentence).  A good question to ask yourself would be: what are these two stories telling us about fear?  That in extreme situations it can lead to madness, even murder?  That it can so warp our thinking that we'll commit the unthinkable in order to avoid facing it?  Once you're able to express this theme/idea in a sentence, you'll have your "topic of discussion" sentence.  Then, tell us how each story is addressing/developing this theme slightly differently.  This will be your thesis statement.