What are some text-based examples of betrayal from Julius Caesar?

In the play, the main examples of betrayal stem from Cassius's betrayal of both Brutus and Caesar as well as Brutus's betrayal of Caesar himself.
In Act 1 Scene II, Cassius betrays Caesar by mocking his physical weaknesses in the hearing of a susceptible Brutus. Cassius relates how he had to carry Caesar out of the River Tiber after the latter tired during a swim.

...so from the waves of Tiber


Did I the tired Caesar. And this man
Is now become a god, and Cassius is
A wretched creature and must bend his body
If Caesar carelessly but nod on him.
He had a fever when he was in Spain,



And when the fit was on him, I did mark


How he did shake. 'Tis true, this god did shake!...
Ye gods, it doth amaze me
A man of such a feeble temper should
So get the start of the majestic world
And bear the palm alone. (Act 1 Scene II)

Cassius's mockery of Caesar is a betrayal or act of disloyalty towards his fellow senator. His main goal is to ensnare Brutus in a scheme to remove the famous Roman general and senator from power.


Brutus and Caesar—what should be in that “Caesar”?
Why should that name be sounded more than yours?
Write them together, yours is as fair a name.
Sound them, it doth become the mouth as well. (Act 1 Scene II)

In using subterfuge to manipulate Brutus, Cassius is also betraying Brutus's trust in him as a friend. In fact, Cassius lies to Brutus about Caesar's supposed ambitions to be king. He slyly suggests that Caesar doesn't deserve the power that he seeks. Yet Caesar is not an overly ambitious man. In Act 1 Scene II, Casca tells Cassius that Caesar thrice rejected the crown that was offered to him. The following text further shows how Cassius means to betray both Brutus and Caesar:


If I were Brutus now and he were Cassius,
He should not humor me. I will this night,
In several hands, in at his windows throw,
As if they came from several citizens,
Writings all tending to the great opinion
That Rome holds of his name, wherein obscurely
Caesar’s ambition shall be glancèd at.
And after this let Caesar seat him sure,
For we will shake him, or worse days endure. (Act 1 Scene II)

Essentially, Cassius uses forged letters to deceive his trusting colleague. The wily conspirator's main objective is to betray Brutus to ignominy and Caesar to his death. One of the most famous lines exemplifying the ultimate betrayal comes from Act III Scene 1:



Et tu, Bruté?—Then fall, Caesar.
(dies)




Here, Brutus is the last to stab Caesar to death. It is the ultimate betrayal because Caesar considers Brutus a trusted friend. Later, Antony cleverly betrays Brutus's trust in him by using his oratorical skills to incite the crowd to violence. You will find evidence of Antony's clever use of apophasis in Act III Scene II, where he skillfully suggests that the conspirators should be implicated for their dishonorable actions against Caesar. Antony tells the crowd that he means to "bury Caesar, not to praise him." However, he volubly praises Caesar in his eulogy.


When that the poor have cried, Caesar hath wept.
Ambition should be made of sterner stuff.
Yet Brutus says he was ambitious,
And Brutus is an honorable man.

Throughout his speech, Antony continues to praise the "honorable" Brutus while implying that his actions are less than honorable. This is what we call apophasis, which is almost identical to praeteritio.

 
So, the above constitutes some good examples of betrayal in Julius Caesar.