Cassius enticed Brutus to become titular leader of the conspiracy. Cassius knew that he himself was not well-liked. He needed someone with an impeccable reputation to serve as figurehead. What Cassius did not anticipate was that Brutus would become unmanageable after Caesar's death and would want everything done his own way--and his own way was sometimes wrong.
Brutus was essentially a good man. He was public spirited, intelligent, kind and thoughtful. But he was a bookish intellectual, an introvert. He was not a man of the people like Marc Antony. He did not understand the selfishness of human nature, since he was unselfish himself. When he is arguing with Cassius in his tent before the Battle of Philippi, he tells him:
I did send to you
For certain sums of gold, which you denied me:
For I can raise no money by vile means:
By heaven, I had rather coin my heart,
And drop my blood for drachmas, than to wring
From the hard hands of peasants their vile trash
By any indirection: I did send
To you for gold to pay my legions,
Which you denied me: was that done like Cassius?
Should I have answer'd Caius Cassius so?
Brutus is telling the truth. Unlike Cassius, he cares nothing about money. It is ironic that he asks, "Was that done like Cassius?" In fact, it was done exactly like Cassius. A good way to find out who your friends are is to try to borrow money from them. Brutus is essentially a philosopher. He is frequently shown in solitude in this play, very much like Hamlet. He even withdraws from his devoted wife Portia when she tries to find out why he is so troubled.
Brutus simply does not understand other people. If he did understand them, he would be appalled by their greed and selfishness. Shakespeare understood people, and he understood Brutus, but Brutus did not understand others. He trusted Marc Antony and permitted him to speak at Caesar's funeral. This was the worst mistake Brutus ever made. Cassius understood the cunning Antony because Cassius was cunning himself. He warned Brutus:
You know not what you do: do not consent
That Antony speak in his funeral:
Know you how much the people may be moved
By that which he will utter?
Brutus by this time had taken it upon himself to become an autocrat. Someone had to fill the power vacuum left by the death of Caesar, and Brutus evidently considered himself the best man for the job. It turned out that by permitting Antony to deliver his famous funeral oration and arouse the Roman people against the conspirators, that it was Antony who filled that power vacuum. Yet Brutus didn't learn his lesson. Cassius had created a monster. He couldn't control him by persuasion, and finally he gave up and went along with Brutus's plans regardless of whether he agreed with them or not.
Brutus resembles other Shakespearean characters in being bookish, introverted, philosophical, solitary, and impractical. These characters include Hamlet, Richard II, Duke Senior in As You Like It, and Prospero in The Tempest, who tells his daughter Miranda:
The government I cast upon my brother
And to my state grew stranger, being transported
And rapt in secret studies.
As the great German philosopher Arthur Schopenhauer observed:
It is just the lower grades of intellectual superiority, such as shrewdness, cunning, and definite but one-sided talents that enable one to get on in the world and readily establish one's good fortune, especially when impudence and effrontery supplement such talents.