Since Cassius and Brutus spend so much time together throughout the play, Shakespeare wanted to differentiate them. Cassius is obviously much more cunning and worldly wise than Brutus. Brutus is portrayed as a reader, a philosopher, and an idealist. He is not necessarily less intelligent than Cassius, but he is less cunning. His main weakness is that he does not understand people. He judges others by himself. He totally misjudges Antony, even though Cassius warns him against Antony repeatedly. When the group of conspirators is discussing the assassination plan at Brutus' home, Cassius says:
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.
But Brutus overrules him. He has agreed to kill Caesar but he doesn't want to look like a "butcher." Then after the assassination of Caesar, Cassius urgently advises Brutus not to allow Antony to speak at Caesar's funeral, but Brutus overrules him again. Brutus is a noble, generous man who judges others by himself. He assumes that all the conspirators were motivated by the same patriotic idealism that motivated him to become their leader. But as Antony says near the end of the play, as he and Octavius stand over Brutus' body:
This was the noblest Roman of them all.
All the conspirators, save only he,
Did that they did in envy of great Caesar;
He only, in a general honest thought
And common good to all, made one of them.
Brutus is a better man than Cassius, but in this world it is not always the honest, decent men who win the prizes. George Orwell shows this truth in his story Animal Farm. The animals get rid of the brutal, greedy human owner of the farm, but then the pigs take over and prove to be as bad as humans, if not worse.
Cassius might never have succeeded in recruiting a group of men to murder Caesar if he had not been able to persuade Brutus to become their leader. And Brutus would never have thought of killing Caesar if Cassius had not talked him into it. This is an example of how two men may be relatively innocuous separately but can form a lethal combination when they join together. Another good example is the deadly partnership of Perry Smith and Dick Hickock in Truman Capote's novel In Cold Blood.