"The fault, dear Brutus, is not in our stars, / But in ourselves, that we are underlings," Cassius says to Brutus when he tries to convince him to join the conspiracy to assassinate Julius Caesar (1.2.141-42). Here we see Cassius reject fate, and Brutus ultimately agrees with him, taking responsibility for keeping Rome “free” by killing Caesar to prevent him from becoming a despot. This is one reason we can understand Brutus as a tragic hero, for if he blamed everything on fate, he would have learn nothing. At the end of the play he chooses to die nobly, on his own sword, because he knows he cannot win and has not brought the freedom to Rome that he thought he would by assassinating Caesar. Antony calls him a great man at the end of the play for this very reason: he did what he thought was right, took responsibility for that, and died for that. Had he attributed everything to fate, he would not have been a great man.
I would argue that Brutus never was a fatalist throughout the play. To ask if he is still a fatalist suggests that at some point he was prepared to succumb to fate. I don't believe that was ever the case. Brutus recognizes the potential shortcomings of a reign by Caesar and decides to take an active part in the plot against him. After realizing that he's made a mistake in killing Caesar, he tries to assert military dominance over those supporters of JC. In my opinion, his suicide was not an act of submission to fate, but rather his attempt to avoid it. Brutus has to realize that Antony and Octavious would probably keep him alive (which would be fate); by killing himself, he's actively changing the future.....certainly not a fatalist.
In the last act, at the battle of Philippi, Brutus realizes all is lost and decides to kill himself with the same sword that he used to kill Caesar rather than be taken as a prisoner through the streets of Rome. Strato holds the sword while Brutus runs upon it.
Brutus has been conflicted since Caesar was killed with whether his motives were as honorable as he first considered them to be, and if the conspirators did the right thing. By Act V, Brutus seems ready to to put an end to this conflict even it means death for him. Some critics feel Brutus makes the military mistakes he does because he's guilt-ridden. There's no doubt he's gone from an optimistic view of his deed to a pessimistic one by the end of the play. Both Octavius and Antony treat Brutus as an honorable man at the end, knowing that he truly was a man of honor and moral principles.