In Act Two, scene one, of Shakespeare's Julius Caesar, the conspirators plan the assassination of Caesar, but they also discuss the fate of Marc Antony. Cassius is shrewd and realistic, believing that Antony should also be killed so he is not a future threat.
...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,
If he improve them, may well stretch so far
As to annoy us all, which to prevent,
Let Antony and Caesar fall together. (162-168)
Brutus, however, disagrees with Cassius. Brutus notes that they are not "butchers." He is concerned that by killing Antony as well, it will prove that Brutus and the others are simply murderers. Brutus asserts that killing Caesar can be (he hopes) perceived as an honorable thing, while killing Antony at the same time will make them seem "angry"—not truly devoted to a cause—which is the case with Brutus, but not with Cassius. If the other Romans see that what these men have done is...
…necessary and not envious… (185)
...and they shall be seen in a favorable light:
We shall be call'd purgers, not murderers. (187)
While Caesar and Antony are close, Brutus argues, Antony is simply one who serves the great Caesar and should be spared.
Our course will seem too bloody, Caius Cassius,
To cut the head off and then hack the limbs
Like wrath in death and envy afterwards;
For Antony is but a limb of Caesar. (169-171)
Let's kill him boldly, but not wrathfully... (178-179)
...This shall make
Our purpose necessary and not envious,
And for Mark Antony, think not of him,
For he can do no more than Caesar's arm
When Caesar's head is off.(184-190)
With regard to Antony's potential as a threat, Cassius is actually right.
After the killing has occurred, Brutus, reputed to be a competent, admired political and military leader, engages in a series of mistakes.
In terms of Brutus' character, he is naive. This behavior is consistent with several of Brutus' actions. He is a truly great man: willing to die for his country and wanting only what is good for Rome—but he is unrealistic.
It is at Brutus's behest that Mark Antony is spared, and it is Brutus who permits Antony to address the crowd after he departs from the scene.
Brutus is naive to believe that Antony will quietly sit by and do nothing after Caesar's murder. He does not consider human nature: that Antony might mourn Caesar, but potentially also want to be named the new Caesar. He believes that Antony (who he does not see as a threat, but simply as Caesar's "arm") will deliver his funeral speeches as promised—without turning the crowd against Brutus and the rest of the assassins.
The primary issues surrounding Brutus's character are his idealism and devotion to the [idea] of republicanism, his political judgment, his motives for joining the conspiracy...
Brutus joins the "conspiracy" believing that Cassius and he are really united in the same purpose—he does not know who Cassius is beneath the surface—jealous and insecure: not in the least concerned about the good of Rome as much as the good of Cassius. Perhaps Brutus believes things can be as he wants them to be, rather than recognizing that the wish does not equate with reality, that the hope is not the same as the fact.