Why does Brutus refuse the second killing?

Brutus refuses the second killing, namely the assassination of Mark Antony, because he believes that their cause is noble and should not be marred by unnecessary bloodshed. He argues that they have already committed a grave act by killing Caesar and that further violence would undermine their justification for the assassination. Brutus prioritizes honor and morality, thinking that sparing Antony would demonstrate their commitment to the ideals of freedom rather than tyranny.