Brutus tells Cassius he's vexed by his own private thoughts.
Brutus hasn't officially been invited to join in the conspiracy against Caesar, but when Cassius approaches him about it, he is already wrapped up in some internal struggle. He tells Cassius he's vexed by private thoughts, likely worrying about what will happen to Rome with Caesar gaining so much power. Brutus then agrees to consider Cassius' plot.
Brutus commits to killing Caesar for the good of Rome.
Brutus reasons with himself that Caesar must be taken down before he can do anything that's actually worth taking him down for. Brutus admits he sees no harm in Caesar now but convinces himself that his transformation into a tyrant is inevitable. By the time anyone can prove Caesar is too ambitious, it'll be too late to do anything about it. Brutus commits to halting Caesar's rise to power through any means necessary.
After murdering Caesar, the people have taken to the streets thinking it's Doomsday. Brutus will speak to the people and convince them of his reason for killing Caesar, and the conspirators expect Antony to confirm it.
Brutus isn't stressed out by all the panic – he's convinced the people will be calmed once they hear his rational, unemotional speech about Caesar's ambition. Further, Antony convinces Brutus that he'll be a friend to the conspirators if he can speak at Caesar's funeral. Though Cassius warns him against it, Brutus leaves after his own speech and lets Antony take the stage to say what he will. Antony's speech undoes Brutus' attempts to quell the people's panic.
Antony has raised the people to mutiny; Brutus cannot even trust his friend Cassius.
The people have risen in mutiny, and Brutus has run away from Rome with Cassius. Brutus is mad when he finds out Cassius has been condoning bribes. If the conspirators act dishonorably, he says, they will undermine their argument that they killed Caesar out of good motives. Antony and Octavius are mounting an army. Basically, Brutus has killed one friend and is being betrayed by another, who is turning out to be a scoundrel. Things are looking pretty dire.
Destruction or Death Wish Stage
Portia has died; Cassius points out that they'll be dragged through the streets of Rome if they're captured; Brutus resigns himself to death by his own hand.
Once it comes out that Brutus' wife Portia has died, it makes sense that Brutus has been so negative. In a speech that's more than a little reminiscent of Macbeth's when he hears that Lady Macbeth has died, Brutus basically says everybody dies, but at least they only die once, so there's no point in dwelling on it. Brutus cares little about his own life. It's clear he can't save Rome from Antony, and his plan against tyranny has failed. Rather than be captured, he clears himself of guilt and avenges his friend Caesar by taking his own life.