The assassination of Julius Caesar and the resulting civil war come about because of envy on the part of Cassius, the idealism of Brutus, and the desire for revenge by Marc Antony.
In Act I, Scene 2, in his hatred for Caesar and desire for his death, Cassius uses flattery and innuendos to convince Brutus that the once-loved Caesar is a tyrant:
....I have heard
Where many of the best respect in Rome
[Except immortal Caesar], speaking of Brutus,
And groaning underneath this age's yoke,
Have wished that noble Brutus had his eyes. (1.2. 60-64)
Further, Cassius expands upon his argument that Brutus should rule because Caesar has lost touch with the Roman people--
And this man
Is now become a god....(1.2.18-19)
He adds that Caesar is like a Colossus, and "we petty men/Walk under his huge legs and peep out...(1.2.137-138)
Brutus adds that Caesar is not even physically well: When Caesar was in Spain, he fell and trembled; his lips lost their color and he groaned and cried for water.
By flattering Brutus, Cassius ingratiates himself to this noble Roman who is his friend in the hope of standing beside Brutus should he become ruler of Rome.
The arguments of Cassius become convincing to a pensive Brutus while he walks in his garden at night and deliberates in Act II, Scene 3. Pondering what Cassius has told him, Brutus wonders if Caesar will be honorable and just if he is given even more authority.
That lowliness is young ambition's ladder,
Whereto the climber upward turns his face;
He then unto the ladder turns his back
Looks in the clouds, scorning the base degrees* [*people in lower positions] (2.1.22-25)
Unfortunately, Brutus is emotionally flawed by his idealism, as well as his ignorance of the nature of the plebeians. For, he believes that Cassius and the others have the same intentions of doing what is good for Rome rather than serving their own needs. He also misjudges Marc Antony, who feigns his respect and support for Brutus and the others, as well as acting as though he desires to maintain order.
In Act IV, Scene 2, Brutus and Cassius, whose relationship has deteriorated, argue about battle plans, and an emotional Cassius offers his dagger to Brutus, expressing his old fault of jealousy,
Strike as thou didst at Caesar; for I know
When though didst hate him worst, thou loved’st him better
Than ever thou loved’st Cassius (4.2.159–161).
In his love for Caesar, Marc Antony pretends that he wishes to give an oratory for Caesar and Brutus agrees, despite Cassius's warning not to allow Antony any forum. When Marc Antony gives his rhetorical orations, he initially says that he comes "to bury Caesar, not to praise him." However, in his emotional state after the death of his beloved Caesar, Antony foments the crowd to civil disorder as he casts aspersions upon Brutus and the others, raising doubts in the crowd as he extols the brave and generous deeds of Caesar, but repeats after each example,
But Brutus says he [Caesar] was ambitious,
And Brutus is an honorable man.
Further, he reads Caesar's will in which he leaves to the plebeians some of his fortune. Then, he manipulates the crowd more and "stir[s]" them to "a sudden flood of mutiny" (2.1.221). Thus, in his hatred for Brutus and the other conspirators, Marc Antony selfishly betrays his love for Caesar by generating a civil war that proves to be devastating to Rome.