When writing on the nature of a tyrant, you may first want to determine if by tyrant you simply mean someone in absolute power or if you more specifically mean a seemingly evil oppressor in absolute power.
Next, since your theme is "nature of a tyrant," you will want to come up with a thesis statement about the nature of a tyrant. Research and read and come up with a statement you believe about the nature of a tyrant that you could support. Some examples follow:
A tyrant's nature is based on the preexisting culture as set forth by his predecessors (this one is still a bit broad and obvious, perhaps).
While tyrants are notoriously evil in nature, many tyrants throughout history have simply been misguided and believed themselves to be acting for the good of their people.
Once you have selected a thesis statement based on your preliminary research, find three to six ideas that support your thesis (based on the length and depth of your paper). Provide examples of tyrants, historical events, and/or sociological tendencies that support your thesis.
Need help with your preliminary research or ideas? Go to the library's Internet database of articles and search "tyrants" to see what comes up.
There are so many different ways to describe the nature of a tyrant. Let me give you a few ways others before have done it; in this way, you will have some examples and perhaps inspiration.
First, Plato talks about the tortured soul of the tyrant in the dialogue Gorgias. Therefore, you might want to weave in your short story the ugly inner qualities of the tyrant. Ask why he or she does what he or she does? What makes them like this? You can be very vivid here.
Second, Tacitus, the Roman historian, also has a lot on the topic of tyranny. For him, Tiberius is a tyrant. In his narrative, he has a section on Tiberius' physical appearance. It is a work of art. Everything about Tiberius, even his good qualities, are slanted in a way that underlines his cruel nature. For example, Tiberius has strong hands. In fact, they are strong enough to crush someone's head.
Based on these two examples, your short story can go in many different directions. I would write a short story about what goes on inside of a tyrant's mind and show how that reflects his outer appearance.
The first thing you want to do is decide your definition of "Tyrant."
A tyrant is, of course, an opressive ruler (what we might think of as a dictator), but not all tyrants are the same. History is filled with opressive leaders, all of which had different temperaments and motivations for their actions. This means that the tyrant or tyrants in your story can come from any number of different perpectives. Ask yourself what sort of tyrant would be most interesting to you? What would you most like to read about?
Once you've made this first step, give yourself a skeletal outline of your story by creating a few major plot points. It may be easy and fun for you to do this by asking yourself a few questions.
Where does your tyrant live and who is under his control? (This will of course give you a fundamental starting point.)
How do those under the control of your tyrant respond to his or her leadership? Do they resist? If so, how? (This will create a conflict which could form the middle, or body, of your story.)
How does your tyrant react to the resistance of the subjects? What are the consequences of his or her actions? (This could be a great place to examine the thoughts and feelings of your tyrant character and also bring your story to a conclusion.)
Once you have a basic outline, use these questions and more like them as a springboard for your thoughts. All that is left now is to let your imagination fill in the blanks!