In Act II scene 1 we see Brutus torn between killing a man, who has no true feelings against, and fighting for the good of Rome. He knows that Caesar wants to be crowned king, and he worries that this power will lead to his corruption, and so he compares Caesar to the egg of a snake, "which, hatched, would as his kind grow mischievous."
Lucius brings his master a letter he found near the window. The letter tells Brutus that accuses him of doing nothing to protect Rome while it is being threatened. He decides that the letter is a sign that the people of Rome are asking him to kill Caesar, and so he accepts their challenge to “speak, strike, redress" or in other words to stand up and fight the wrongs.
This shows us that even though Brutus is planning to kill and overthrow the leader of Rome, he is doing it because he cares about his county and believes he is doing the right thing for the Roman people.