In Shakespeare's play Julius Caesar, the only two ways in which Antony can be said to have betrayed Caesar were, first, in making friends with all the conspirators and shaking their bloody hands. At that time he had no idea that he was going to be able to turn the people against the conspirators. He was just one lone individual with his own life at stake. He did not even know he could succeed in doing anything so seemingly miraculous when he received Brutus's permission to deliver a funeral oration. Presumably, if he had not been successful with his funeral oration he would have become a member of the new government along with Brutus and Cassius. It was during the speech itself, as he observed the mob's reactions to his words and the sight of Caesar's mutilated body, that he began to realize he could turn the tables on Brutus, Cassius and the others.

Then when Antony had achieved power in Rome (along with Octavius and Lepidus), he can be said to have betrayed the dead Caesar by not following up on Caesar's plans to improve working and living conditions for the poor and the working people of Rome. We see Act 4, Scene 1 that Antony is already planning to violate Caesar's will.

But Lepidus, go you to Caesar's house;
Fetch the will hither, and we shall determine
How to cut off some charge in legacies.

This is how Antony is honoring his great friend. In fact, the triumvirate became very unpopular because of their failure to honor Caesar's bequests to the Roman people and for numerous other reasons. Shakespeare acknowledges this historical fact in his play. When Antony urges Octavius to join him in preparing to fight Brutus and Cassius, Octavius replies:

Let us do so, for we are at the stake
And bayed about with many enemies;
And some that smile have in their hearts, I fear,
Millions of mischiefs.    (4.1)

Marc Antony was a complex character. He was most concerned about looking out for himself.

Mark Antony was generally a close friend and confidant of Julius Caesar. Antony joined Caesar’s staff and was a member of his army. Caesar appointed him to military leadership, a role he performed successfully. Even though Antony was an exemplary military leader, he lacked skills as a social leader because of his personality.

After Caesar defeated Pompey to establish the Roman Empire, he appointed Antony as his second in command. Antony betrayed Caesar’s trust by using his power to indulge in extravagance which displeased the citizens who were still suffering the effects of the civil war and thus tainted the administration's reputation.

Antony also appropriated property belonging to Pompey and lied to Caesar that he had bought it. Caesar sought payment for the property. Antony refused to pay, resulting in violence and anarchy in Rome. This betrayal forced Caesar to strip Antony of all political responsibilities.