At the beginning of this scene, Brutus is uneasy about Cassius's loyalty. When Pindarus comes to greet Brutus on behalf of Cassius, Brutus expresses some misgivings to him. Then Brutus pulls Lucillius aside to ask how Cassius has treated him. Lucillius responds that Cassius was courteous and respectful but not "free and friendly" the way he used to be. This worries Brutus, who says to Lucillius:
Thou hast describedA hot friend cooling. Ever note, Lucillius,When love begins to sicken and decay,It useth an enforcèd ceremony.There are no tricks in plain and simple faith.
Brutus is uneasy because he has learned of his wife's death. This is enough to make a man wish for death. He is restless in his situation.
Also, he desires gold to pay his soldiers. In this, he feels that Cassius has betrayed him as well.
Brutus is restless because his precious wife Portia has committed suicide. In so many ways, he feels this is all his fault.
His country has been plunged into a civil war due to the assassination of Caesar. Brutus again feels totally responsible.
While killing Caesar was supposed to free him, Brutus is bound more than ever.
Possibly he questions whether or not he did the right thing in killing Caesar.
If only he will think on Caesar's last words, he will be encouraged in knowing he did the right thing in assassinating Caesar.
"Et tu, Brute? Then fall Caesar." These last words indicate that even Caesar is convinced he should fall if his precious Brute is in on the assassination. Caesar has that much confidence in Brutus. If Brute is in on the killing, then Caesar agrees that he must die.