Why did Brutus kill Cesar

The main reason why Brutus believes that Caesar must die has to do with the people crowning Caesar as king; he believes that Caesar will have too much power, and may use this power in ways that might not best benefit Rome.

Explanation:

Brutus contemplates murdering Caesar and describes his motivation to join the conspirators during his soliloquy in act 2, scene 1. Brutus concludes that Caesar's ambition could influence him to abuse his power and likens Caesar to a "serpent's egg." In order to prevent Caesar from becoming king, disbanding the Senate, and enslaving the population, Brutus feels that it is necessary to kill him.

After Brutus and the senators brutally murder Julius Caesar at the Capitol, Brutus speaks before a large crowd and explains his actions. Brutus argues that Caesar was an ambitious man and describes his motives by saying the following:

If then that friend demand why Brutus rose against Caesar, this is my not that I loved Caesar less, but that I loved Rome more. Had your rather Caesar were living and die all slaves, than that Caesar were dead, to live all free men?

Brutus feels that assassinating Julius Caesar was justified to protect the Roman citizens from tyranny. He is the only senator who does not act out of self-interest and is genuinely concerned about the well-being of Rome. One could conclude that Brutus's love for Rome motivated him to take Julius Caesar's life.