I don't see any irony or find it surprising that Brutus took his own life. For the Romans, to die honorably was very important. If Brutus had lived, he most likely would have been captured and imprisoned. Roman generals had the habit of bragging about their conquests with huge parades in Rome. They would display whatever treasure they had brought back, including prisoners of war, who would have been shackled in chains and marched in the parade. Then they would have been executed. Brutus escaped this dishonor by killing himself.