Brutus lacks knowledge of people and their abilities (one of the themes in the play). He makes incorrect decisions throughout the play and thus displays a flaw in his character that would eventually cost him his life and devastate Rome. He describes Mark Antony as someone who simply supports Caesar and who will not act on his own. Mark Antony is a mere 'limb' of Caesar implying that he lacks the ability to think and act on his own. Furthermore, he believes that killing Mark Antony will lead to too much blood and killing which would make their attack on Cassius appear bloodthirsty and not acceptable to the Romans. Brutus still believes that killing Caesar is a noble act, a sacrifice made to save Rome.
Cassius actually points out how powerful Mark Antony could be, but he is dismissed by Caesar. Brutus is proven wrong.
CASSIUS: Decius, well urged. I think it is not meet
Mark Antony, so well beloved of Caesar,
Should outlive Caesar. We shall find of him
A shrewd contriver; and you know his means,(165)
If he improve them, may well stretch so far
As to annoy us all, which to prevent,
Let Antony and Caesar fall together.
BRUTUS: Our course will seem too bloody, Caius Cassius,
To cut the head off and then hack the limbs(170)
Like wrath in death and envy afterwards;
For Antony is but a limb of Caesar.
Let us be sacrificers, but not butchers, Caius.