Mark Antony, a Caesar loyalist and sympathizer, asks the conspirators to speak at Caesar's funeral. Brutus, who is trusting by nature, agrees to allow Mark Antony to address the Romans even though Cassius warns Brutus that this decision isn't wise.
Brutus, in 3.1.263-267, tells Antony that allowing Antony to speak will make the conspirators' murder seem less brutal to the Romans:
What Antony shall speak I will protest
He speaks by leave and by permission,
And that we are contented Caesar shall
Have all true rites and lawful ceremonies.
It shall advantage more than do us wrong.
On the other hand, Cassius suggests that Antony will do harm instead of good (3.1.258-259):
Know you how much the people may be moved
By that which he will utter?
Ultimately, Brutus's generosity and trust account for his downfall, since the Romans do, in fact, turn against the conspirators because of what Mark Antony says at Caesar's funeral.