In Shakespeare's classic play Julius Caesar, the conspirators are more than willing to commit regicide in order to preserve the Roman Republic and prevent Caesar from becoming the emperor. Although regicide is defined as the action of killing a king and Julius Caesar has not been crowned, the conspiring senators have no qualms about assassinating him on the Senate floor. Cassius is the leader of the conspirators and mentions to Brutus that he will never allow Caesar to tyrannize or oppress him. Cassius encourages Brutus to join their cause by saying,

Men at some time are masters of their fates.
The fault, dear Brutus, is not in our stars,
But in ourselves, that we are underlings (1.2.139-141).

Unfortunately, Brutus is not aware that Cassius and the other conspirators are selfishly motivated to assassinate Caesar and view this as an opportunity to gain political power.

Brutus is depicted as the most honorable, noble conspirator and hesitates to join their cause. However, Brutus ends up convincing himself that Caesar is similar to a "serpent's egg," which is will eventually hatch and "grow mischievous." Brutus goes along with the conspirators' plan to assassinate Julius Caesar and reveals that he is in favor of committing regicide when he says,

My ancestors did from the streets of Rome
The Tarquin drive, when he was call'd a king (Shakespeare, 2.1.53-54).

Similar to his valiant ancestors, Brutus feels obligated to protect the citizens of Rome from tyranny, which is why he agrees to join the conspirators and assassinate Julius Caesar.