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.
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.