When it seems evident to the conspirators in Shakespeare's play that Julius Caesar is headed for absolute power, he becomes a threat to the ideals and values of the Roman Republic. They assassinate Caesar before he can be crowned king. The irony is that Caesar's death results in civil war. As two factions with questionable motives grab for power, chaos ensues and the Republic is never the same again.
By dramatizing the historical circumstances surrounding Caesar's assassination, Shakespeare asks a series of questions relevant to his 16th-century audience and readers today: How should cities and countries be governed? What makes a good leader? What happens when a political leader's power is unchecked? And, what happens when the leader dies without a suitable replacement lined up?
Although Julius Caesar is certainly arrogant and power hungry, there's no clear evidence in Shakespeare's play that suggests he's a tyrant when he's assassinated. Rather, the conspirators see in Caesar a potential threat to Rome and eliminate him before his ambitions can be realized.
Shakespeare's portrayal of the chaos that ensues after Caesar's assassination dramatizes a major late 16th century political concern – what would happen when England's aging monarch (Queen Elizabeth I) died without an heir to the throne?