In 334 B.C. he returned to Athens and founded his own school the Lyceum.  In the school he taught various subjects that included logics, ethics, biology, physics etc.

After Alexander died in 323 BC he was sentenced to death by the Athenians just as his teacher Socrates had been done earlier.  It is believed that Aristotle was sentenced to death more out of resentment for him being the educator of Alexander the Great than anything else.  Alexander had conquered Athens and was highly resented in the city.  The death of Alexander gave an opportunity to some citizens in Athens to revenge themselves by killing his teacher.

Aristotle fled from Athens upon being sentenced to death.  He felt that Athens the city of Democracy should not have its' hands dirtied twice against Philosophy.  The first violation being the murder of his teacher, the second  would have been his murder.

Like his teacher Plato in his philosophy he treated the question of the "ideal state".  However, Aristotle was much more flexible and understanding of mankind than Plato was.  Aristotle followed a different form of knowledge which attempted to understand why certain laws existed effectively while others did not.  Yet like his teacher he believed that the "ideal state" could only be found in the "Polis" the city state.

Aristotle believed that what ultimately mattered was balance between Democracy and Oligarchy.  Like his teacher Plato he believed that the leaders and rulers of governments should be allowed to come from all classes.  To Aristotle the preferred citizens who should rule were those who were just and friendly.  He said that these elements existed mostly in the poorer and middle classes, and therefore these classes should be included more in the government.  He believed that when these classes were removed by the "rich" class in a society  the end result was always revolution.

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Look at a quote from Aristotle: Ancient Greek Quote