What are the reasons did Brutus gave to Antony that he will not kill him

When Antony begs of his death, Brutus says that they might look cruel at the moment but he does not know their reason for killing Caesar. He says that their daggers have lost their sharpness to kill anyone else, their hands do not have the strength to kill Antony. They do not have a grudge against him and they accept him as their brother with open hearts.