Why does Brutus join the conspiracy?

He fears Caesar will become a tyrant and believes it is for Rome's good

What fear in 1599 links to the play?

Civil war, because Elizabeth I had no heir

Julius Caesar returns to Rome in triumph. A soothsayer warns him to beware the Ides of March, and Cassius persuades the honourable Brutus that Caesar is becoming too powerful.
Caesar ignores warnings, including his wife Calpurnia's dream, and is stabbed to death by the conspirators on the Ides of March (15 March).
Brutus explains the killing to the crowd, but Mark Antony's funeral speech turns the people against the conspirators.
Civil war follows. At Philippi, Antony and Octavius win; Cassius and Brutus kill themselves, and Antony praises Brutus as the noblest Roman of all.

Ambition and power, honour, loyalty and betrayal, persuasion and rhetoric, fate and free will.
Written in 1599, when the ageing Elizabeth I had no heir and people feared civil war. Shakespeare used Plutarch's Lives as a source.

A tragedy whose hero is arguably Brutus: a good man whose honour leads him to a terrible act.

What does the soothsayer warn Caesar about?: The Ides of March: It is 15 March.
Who persuades Brutus to join the conspiracy?: Cassius: He uses flattery and forged letters.
Why does Brutus join the conspiracy?: He fears Caesar will become a tyrant and believes it is for Rome's good: Antony later calls him the noblest Roman.
Who warns Caesar after a frightening dream?: Calpurnia: Caesar is persuaded to go to the Capitol anyway.
What turns the crowd against the conspirators?: Mark Antony's funeral speech: It is a masterpiece of rhetoric.
Where are Brutus and Cassius defeated?: Philippi: Both take their own lives.
Which theme does Antony's funeral speech show?: The power of persuasion: He turns the crowd while seeming to praise Brutus.
What fear in 1599 links to the play?: Civil war, because Elizabeth I had no heir: The play shows the chaos after a leader is killed.