Three decisions that lead to Brutus's downfall include his involvement in the conspiracy to kill Caesar, his refusal to kill Mark Antony, and the fact that he allowed Antony to address the people of Rome at Caesar's funeral. All of these mistakes arise out of Brutus's sense of honor.
Cassius knows Brutus well, and he ruthlessly manipulates him into joining the plot to murder Caesar by appealing to his sense of honor. Cassius himself has no sense of honor: he wants to kill Caesar because he is envious that his former friend and equal is breaking away and becoming the greater man. He doesn't want to have to bow down to him, and so he would rather see him dead. Cassius calculates that having someone with Brutus's untarnished reputation on the conspirators' side will help convince the people of Rome that the assassins' motives are noble and pure. Tragically, Brutus falls for the callous manipulation. He should never have joined the plot.
Once in, the same sense of honor that caused him to join in with bad business becomes his undoing. Cassius knows that the conspirators should murder Antony as well as Caesar, because Antony will be a dire threat to them. However, Brutus refuses this step as unnecessary and overly violent. This is his second main blunder, again based on his innate sense of decency.
Finally, Brutus makes a terrible and almost senseless blunder when he allows Antony to address the mob merely with the guarantee that Antony won't attack the conspirators. Antony does not attack the assassins overtly, but he uses sarcasm and emotional appeals to turn the crowd against Brutus and his cohort, starting a civil war.
Brutus's blunders, ironically, emerge from his genuine goodness as a human being. Because he is an upright and honorable man, he is blind to how low the people around him will sink in their pursuit of their own agendas. He can't see that everyone in the world is not like him.
As an aside, another poor decision that Brutus makes is wearying the soldiers with the long march before the final battle, but it could be argued that by time, the die had long been cast by Brutus's prior decisions.
Brutus's first terrible decision was choosing not to Cassius when he suggested that they kill Mark Antony. Cassius realizes that Antony is fiercely loyal to Julius Caesar and believes that he will seek revenge. However, Brutus does not want the Senators to be perceived as bloody butchers so he makes the costly decision to allow Antony to live.
Brutus's second costly decision that leads to his downfall was allowing Antony to speak at Caesar's funeral. Brutus does not view Antony as a threat and believes that Antony will pacify the crowd's possible anger. He does not anticipate Antony's moving funeral oration, which incites the masses to revolt against the Senators and make the political situation worse.
Brutus's third tragic decision concerns his plan to meet Octavius and Antony at Philippi....
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In act 4, scene 3, Brutus tells Cassius that he plans on marching to Philippi. Cassius disagrees and mentions that they would waste provisions and tire their soldiers out before the battle. However, Brutus believes that they will receive additional reinforcement troops when they arrive at Philippi, and so he makes the terrible decision to travel before the final battle.
I would argue that Brutus' decision to join the conspirators to kill Caesar was the righ t decision because he truly believed it was for the good of Rome.
His first mistake was allowing Antony to talk at Caesar's funeral and especially leaving him alone without having anyone there to censure him. I think he was correct in keeping Antony alive , for if they did decide to kill Antony too, they would have looked like murderers moreseso than liberators, since Antony was not nearly powerful enough to warrant such an assassination.
His third great mistake was NOT listening to Cassius in Act V and instead marching to Pompey to meet the enemy. Cassius was correct in his assumption that the soldiers would be weary and unable to fight well after such a long trek.
The first decision that led to Brutus' downfall is when he decided that he would join the conspirators in their plot to kill Caesar. The plot did not work. The conspirators had to flee for their very lives.
The second decision that led to Brutus' downfall is when he refused to listen to Cassius. Cassius warned Brutus to kill Antony along with Caesar. Brutus did not want the conspirators to appear as butchers so he allowed Antony to live.
The next decision that led to Brutus' downfall is when he gave Antony permission to speak at Caesar's funeral. Again, Cassius warned Brutus not to allow Antony to speak. Cassius feared that Antony would turn the people against the conspirators, and that is exactly what happened. Brutus had to flee for his life which ultimately led to his death.