What reasons does Brutus give for killing Caesar in his act 2 soliloquy in Julius Caesar?

Brutus' monologue is inspired by the conversation he has had with Cassius in Act l in which the latter has spoken openly about his resentment for Caesar and the power he has attained. Cassius has pointed out Caesar's weaknesses and questioned his ability to lead. He believes that they are not only his equals but have greater power and authority to lead. He has made it clear that he and other conspirators have already begun plotting Caesar's downfall and want Brutus, who is trusted by Caesar, to join them.

In his monologue, Brutus expresses the sentiment that there is no other way to get rid of Caesar than by assassination. He states that he has no personal grievance against the general but that it would be for the greater good. Brutus is aware that the general populace wants Caesar crowned and that it would, therefore, happen. He is, however, afraid of what effect such entitlement will have on Caesar's character and metaphorically surmises that it might bring out the worst in him:

...It is the bright day that brings forth the adder...

It is this aspect, he suggests, that should encourage caution and greater awareness. Brutus questions the idea of crowning Caesar for he believes that giving him so much power might make him become pernicious. He may, without constraint, commit evil. He surmises that power is abused because it separates its bearer from feeling any remorse. He does accede, though, that he has never seen Caesar being swayed so much by emotion that he became irrational, thus suggesting that the general is of a stable nature.

Brutus assumes that it is generally true, though, that those with low rank will be driven by ambition to achieve better and aim for the greatest authority, but once they have attained such power, would look only toward their greatness and forget and despise the manner and method by which they had actually achieved their goal. He believes that Caesar may do the same and, because he might become too arrogant, it is best to prevent him doing so.

Brutus then suggests that, since their opposition to Caesar cannot be justified in these terms at the moment, as he has not displayed any of the negative traits mentioned, it would be best to argue that, should Caesar be given greater power and authority, he would become abusive. He believes, therefore, that Caesar should be seen as an unborn serpent, one with the capacity to do great mischief. Caesar's power would give him the authority to do great harm, as it is natural for those such as he to do so. It is, therefore, imperative that he should be killed 'in the shell' and prevent a monster from being born.

Why does Brutus join the conspiracy against Caesar, and how do his motivations compare to Cassius's?

Brutus is manipulated by Cassius into believing that Julius Caesar is an ambitious man, who poses an extraordinary threat to the Roman Republic. Being one of Caesar's closest friends, Brutus is initially hesitant to join the group of conspirators. After seriously contemplating whether or not he should participate in Caesar's assassination, Brutus ends up reading false letters from concerned citizens and decides to join the conspiring senators. Unlike Cassius, who is motivated by greed and the opportunity to advance his political power, Brutus chooses to assassinate Julius Caesar in order to protect the Roman Republic. Brutus sincerely believes that Caesar desires to become emperor, wishes to disband the Republic, and ultimately rule Rome as a tyrant. Essentially, Brutus has honorable intentions and does not wish to personally benefit from Caesar's death like Cassius.

Despite Brutus's positive intentions, there is not much evidence during the play to suggest that Caesar wishes to definitively rule Rome as an emperor. Overall, Cassius successfully manipulates Brutus into perceiving Caesar as the ultimate threat to the Roman population through moving arguments, which motivates Brutus to act honorably and defend Rome by joining the conspirators. Unfortunately, Brutus makes several tragic decisions that lead to a civil war, which is won by Octavius, Antony, and Lepidus.

We can't really know if Brutus's actions are right or not.  The only way to tell would have been to let Caesar live and see if he truly became a tyrant.  Brutus thought he was going to, but it's a little iffy (in my opinion) to assassinate someone because of what they might become.  So I think that Brutus had good motives, but there is no way to know if he was right to join the conspiracy.

What were the motives of Brutus and Cassius for assassinating Caesar?

In Shakespeare's Julius Caesar, the two conspirators Cassius and Brutus have drastically different reasons for planning to kill Caesar.

Cassius's main reason for planning Caesar's murder is his envy of Caesar's power. Cassius believes that he is just as fit, if not more so, to rule Rome, and his desire to kill Caesar stems from this belief. At one point of the play, Cassius recalls a time during childhood in which Caesar nearly drowned in the river and Cassius saved him. Cassius pays specific attention to the image of Rome's mighty leader drowning amongst the torrents, and he cites bitter anguish at his political power:

And this man
Is now become a god, and Cassius is
A wretched creature and must bend his body,
If Caesar carelessly but nod on him.

Brutus's main reason for killing Caesar is his own undying love for Rome. He becomes entirely torn between these two loves but is ultimately convinced by Cassius that Rome is the more important of the two. His reluctance to betray his closest friend is slowly worn away.

Both of these two men die shortly after killing Caesar, as the assassination sets forth an onslaught of violence.

Cassius appears to envy Julius Caesar, while Brutus is fearful for the Roman Republic. Cassius points out that Caesar has grown too powerful, though he is not more worthy than anyone else: “Brutus and Caesar: what should be in that 'Caesar'? / Why should that name be sounded more than yours?” Cassius condemns Caesar’s weakness as a human, emphasizing that he is nothing more than a man who may aspire to be dictator. Caesar describes Cassius’s ambition and jealousy: “Such men as he be never at heart's ease / Whiles they behold a greater than themselves.”

Brutus, on the other hand, cares about Caesar: “I know no personal cause to spurn at him, / But for the general.” However, he worries about how power will corrupt Caesar and believes it necessary to nip his aspirations in the bud. At Caesar’s funeral, Brutus emphasizes how much he loved and mourns for Caesar: “Not that I loved Caesar less, but that I loved Rome more.” He slew Caesar for his ambition, which he considered a threat to Rome. As far as Brutus is concerned, a monarchy would make them all slaves.

Both Cassius and Brutus worried about Caesar’s growing popularity. Caesar was a proud and strong-willed man, so their fears were very valid. Their conflicting motives result in some irreconcilable contradictions in their coup, which ultimately falls to Mark Antony and Octavius Caesar. Octavius would usher in the Roman Empire, the exact fate Brutus was hoping to avoid.

Compare the motives of Cassius and Brutus for assassinating Caesar.

Cassius and Brutus have completely different reasons for participating in Julius Caesar's assassination. Cassius is portrayed as a shrewd politician who views Caesar as a threat to his position of authority and influence in the Roman Senate. Cassius fears that Caesar will become an authoritarian emperor and disband the Senate. He also views Caesar's death as an opportunity to advance his political status and attain more authority and wealth. Cassius's true intentions are revealed following Caesar's assassination when he begins to sell political offices and receive bribes. Essentially, Cassius has selfish motivations for joining the conspirators and assassinating Julius Caesar.

Unlike Cassius, Brutus is depicted as an honorable man who joins the conspirators to prevent Caesar from ruling Rome as a tyrant. Brutus struggles with his conscience regarding his participation in Caesar's assassination and ends up comparing Caesar to an unhatched serpent's egg, which is potentially dangerous. Brutus is manipulated by Cassius and convinces himself that Caesar is an ambitious man with selfish intentions. Brutus is the only senator who has noble intentions and is truly concerned about the state of Rome. He does not want Caesar to tyrannize over Rome, so he assassinates his close friend to save the Roman populace.

What motivates Cassius to lead the assassination plot against Julius Caesar in Julius Caesar?

Do not overlook one of the strongest and most insidious of motivators in human nature:  ENVY.  Caesar himself senses this dangerous quality of Cassius as he and Marc Antony and the "train" enter the streets of Rome:

Let me have men about me that are fat,/Sleek-headed men, and such as sleep a-nights,/Yond cassius has a lean and hungry look;/He thinks too much:  such men are dangerous....Ge us a great observer, and he looks/Quite through the deeds [sees through their motives]of men....Seldom he smiles, and smiles in such a sort/As if he mocked himself, and scorned his spirit/That could be moved to smile at anything./Such men as he be never at heart's ease/Whiles they behold a greater than themselves,/And therefore are they very dangerous. (I,ii,192-210)

That Cassius is envious of Caesar's power is evident in his words.  He speaks to Brutus of being ignored by him, showing his desire for attention and affection:

Brutus, I do observe you now of late;/I have not from your eyes that gentleness/And show of love as I was wont to have....(I,ii,32-34)

In what is known as "the seduction scene" of Brutus by Cassius, Cassius offers no concrete evidence of Caesar's tyranny or ambitions except for describing Caesar as a Colossus, but even in this description, the envious nature of Cassius is evidenced in such words as "we petty men."

Why, man, he doth bestride the narrow world/Like a Colossus, and we petty men/Walk under his huge legs and peep about/To find ourselves dishonorable graves (I,ii,135-137)

Cassius tells Brutus how he, who "was born free as Caesar" I,ii,97) had to save Caesar, who became weak, from drowning; yet, this same Caesar

Is now become a god, and Cassius is/A wretched creature, and must bend his body/If Caesar carelessly but nod on him. (I,ii,116-117)

In his envy of Caesar's power, Cassius manipulates his brother-in-law, "seduces" him by means of flattery and an appeal to Brutus's sense of honor and republican ideals because he knows that the Romans respect Brutus who will be more able to effect a change of power. In his famous remark to Brutus, he all but says "why not me?":

Men at some time are masters of their fates:/The fault, dear Brutus, is not in our stars,/But in ourselves, that we are underlings./Brutus (and me!) and Caesar: what should be in that "Caesar"? (I,ii,139-141)

(When you have Cassius in court, you could question him about his words:  Isn't it true, Cassius, that on ----you said, ----?  Did you not remark to Brutus that -------?  Make Brutus testify, too, if you can in order to have Brutus verify what Cassius has said.)

This sounds fascinating and very challenging.  I am confident that you will be pushed with this assignment.  It might be best for you to identify three reasons why Cassius was motivated to plot against Caesar  (Juries and audiences remember three reasons.)

1)  The belief that Caesar sought to subvert the republic into his own identity- Cassius lays out the case in the "seduction scene" to enlist Brutus' help that Caesar has ambitions that are larger than the republic.  His main argument is that continued rule by Caesar will lead to him gaining in stature so much so that a dictatorship will result.  Cassius' main intention here is to sway Brutus that removing Caesar from power is a good thing to help the republic prosper.

2) Caesar is not physically able to carry out the demands of being the architect of the republic- In another tact in this scene, Cassius suggests that Caesar is not able to carry out his duties from a physical sense and that this weakness will be read by enemies as the weakness of the republic.  Again, invoking notions of the republic, removing Caesar becomes a critical issue.

3)  Greatness is defined by individuals who seize the moment- This analysis is more aligned by his character.  I think you want to find instances in the first half of the play where Cassius talks about the greatness of men, and the heights that great men can achieve.  He extols his own virtues in trying to enlist Brutus' help and Cassius is a believer of free will (Note his line in that the "fault is not in the stars.")  Greatness in history is defined by the actions of the great ones who seize and own the moment.  He points to Aeneas' founding of Rome and then suggests to Brutus that this is their moment.

Going back and scouring the text for moments that substantiate these reasons will give a portrait of someone who possessed ambition and was willing to do what was necessary in the achievement of his goals.

What motivates the conspirators to assassinate Caesar in "Julius Caesar"?

The conspirators think that Caesar wants to destroy the Roman Republic and make himself king. He's already made himself dictator for life, so the next logical step is to go one better and have himself crowned, just like the Etruscan kings of old. Brutus and the other conspirators deeply venerate the Republic, which was founded after the Etruscan king Tarquin the Proud was overthrown in a popular uprising. From that day on, the very idea of kingship was anathema to the Romans, and woe betide anyone who gave the slightest impression that they wanted to revive the old Etruscan custom.

Caesar disavowed the crown in a very public show of forbearance, but none of the conspirators erre buying it. To them, it was nothing more than a gigantic charade designed to impress the plebs with Caesar's supposed modesty. They believe they know what Caesar really has in mind; they're stone-cold certain that he wants to make himself king and turn the Roman people into slaves. And the only way they can prevent this terrible fate from befalling the Republic is by having Caesar assassinated.

The conspirators view Julius Caesar as a threat to their positions of authority and fear that he will become emperor and disband the Senate. If Caesar were to become emperor, the senators' livelihoods would cease to exist, and they would lose their authority and influence in Rome. The conspirators also wish to increase their political power and wealth by eliminating Julius Caesar from his prominent position. Following Caesar's assassination, Cassius reveals that he was not concerned at all about the Roman populace and was instead selfishly motivated to kill Julius Caesar. Cassius proceeds to demonstrate his selfishness by selling political offices and accepting brides.

Out of all the conspirators involved in Caesar's assassination, only Brutus has purely honorable intentions. Brutus joins the conspirators because he fears that Caesar will rule Rome as a tyrant. Brutus likens Caesar to a "serpent's egg" and believes that his ambition will motivate him to rule as emperor (after Cassius has planted this idea in his mind). Unlike the other conspirators, Brutus is not selfishly motivated; he participates in the assassination in an attempt to protect the Roman populace from tyranny.

In short, Caesar's ambition is the impetus of the assassination plot.  His desire to become king is believed by the group of would be assassins to be an imminent threat to the current way of life.  Caesar's rule would overturn the republic, and Cassius concurs that Caesar is treated like a god though he is merely a man.  

What reason does Brutus provide for Caesar's death?

In act 3, scene 2, Brutus addresses the crowd at Julius Caesar's funeral to explain why the senators assassinated Caesar, in an attempt to quell the emotional citizens. Brutus begins by asking the citizens to believe that he had honorable intentions and encourages them to judge him fairly after listening to his explanation. Brutus then tells the crowd,

If then that friend demand why Brutus rose against Caesar, this is my answer: not that I loved Caesar less, but that I loved Rome more (Shakespeare, 3.2.18-20).

Brutus continues by asking the citizens if they would rather be slaves because Caesar were living or enjoy their freedom because Caesar is dead. Brutus then directly tells the crowd that it was Caesar's ambition that motivated Brutus to join the conspirators and assassinate Caesar. Brutus says,

But, as he was ambitious, I slew him. There is tears for his love, joy for his fortune, honor for his valor, and death for his ambition (Shakespeare, 3.2.25-27).

The citizens initially accept Brutus's explanation and praise him for saving them from possible tyranny. However, Brutus makes the significant mistake of allowing Mark Antony to deliver his funeral oration. Antony ends up giving a moving, passionate speech, which stirs the crowd to riot against the senators and leads to the Roman civil war.

How do Cassius' motivations compare to Brutus' in Julius Caesar?

Brutus' character is summed up at the end of the play in a speech by Marc Antony:

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.
His life was gentle, and the elements
So mix'd him that Nature might stand up
And say to all the world: "This was a man!"
(V.v.68-75).

The very phrase "this was a man" echoes the "ecce homo" that would have been inscribed on crosses in the churches Shakespeare attended.

Cassius, on the other hand. has a "lean and hungry look," a hunger not for food per se but for power. His defense of the Republic is motivated to a degree by the way in which he himself and other members of the Senate had more power under a monarch than they would have under Julius Caesar as Emperor. While Brutus acts from pure benevolence, and is genuinely sorrowful at the death of Caesar, Cassius is lacking in remorse and more self-interested.

Shakespeare’s Julius Caesar is based on actual events which occurred during the assassination of Caesar.  The two primary conspirators were Cassius and Brutus. Each man had different reasons for wanting Caesar not to become the Emperor of Rome.

On February 15, 44 B.C., Cassius confronts Brutus with his reasons for despising Caesar.  He knows that he needs Brutus to join the conspiracy because of his popularity with the other senators and the people.

Cassius outlines the times that he has seen Caesar show weakness in his actions. He also knows that something is bothering Brutus. When Brutus hears the cheers of the crowd, Cassius notes that Brutus is afraid that Caesar is being offered the crown.  This inspires Cassius to share his feelings about Caesar. 

Cassius lists three instances that he believes indicate Caesar’s weaknesses.

1st incident

Caesar challenges Cassius to a swimming contest. He dares Cassius to swim across the Tiber in rough waters to a point across the river.  Both of them jump in with their full armor on.  Half across the water, Caesar cries out for help: “Help me, Cassius, or I sink!”  Cassius pulled Caesar from the rough water.  Caesar was exhausted. Cassius cannot imagine that now this man is going to become a deity?

2nd incident

Caesar has a fever when he was in Spain.  Later,  he has a fit [seizure], and Cassius noticed how Caesar shook..  He lost the color in his lips, and his eyes became dull.  The tongue that will rule the world cried and groaned.

3rd incident

When Caesar was sick, he cried out as though he were a child.  He begged for water as though he were a girl. It is shameful that this weak man is going to be the supreme ruler of Rome.

Brutus tells Cassius that he must think about what Cassius has said and come to his own conclusions.  

Brutus decides that Caesar must die  not for personal reasons but for the good of the Republic.

When Brutus is seen again, it is on March 14, 44 B.C. Brutus has spent many sleepless nights determining what he should do.  Brutus has no personal problems with Caesar.  He is concerned how power would change Caesar. Brutus decides that Caesar must die,  not for personal reasons but for the good of Rome. He provides three analogies comparing the possibilities of Caesar and his rule.

1st Analogy

If Caesar were a snake and it was out in the daytime, it would make sense to avoid the poisonous snake to keep from being stung.

How would the crown change Caesar? Would he become like the snake?

 2nd Analogy

Brutus has never known Caesar to misuse his power. On the other hand if he were to climb the ladder of success, people would support him on the way up toward his goal. The climber looks toward the tops trying to ascend the ladder. When he gets to the top, the person often forgets those who have helped him along the way. Caesar might do this. 

3rd Analogy

What if Caesar were like the snake that is still in the egg? When the snake hatches, it becomes dangerous. Rather than allow the danger to occur, kill the snake while it is in the shell. Then, it will cause no problems. This is what must happen to Caesar.

And therefore think him as a serpent's egg
Which hatch'd would as his kind grow mischievous,
And kill him in the shell

Brutus bases his decision on the potential changes that Caesar might make. It is the possibilities not the realities that scare Brutus into joining the conspiracy.

What reasons does Brutus give for Caesar's assassination in his speech?

In Shakespeare's Julius Caesar, Brutus appeals to the reason of the Roman crowd.  He offers himself over to their judgement, and presents them with a rational argument. 

He begins by establishing that he was as much of and even more of a friend of Caesar's as any one present:

...If
there be any in this assembly, any dear friend of
Caesar's, to him I say that Brutus' love to Caesar was
no less than his.  If then that friend demand why
Brutus rose against Caesar, this is my answer:  Not
that I loved Caesar less, but that I loved Rome more.  (Act 3.2.17-22)

The implication is that if Brutus loved Caesar this much, he must have had good reason for assassinating him.

This is a powerful opening, but at this point Brutus makes his mistake.  He tells the crowd that Caesar was ambitious, that had Caesar lived they would have all been slaves.  Everything else he says comes down to Caesar's ambition.  But his mistake is that he doesn't prove Caesar was ambitious.  He tells the crowd his conclusion about Caesar's character, but he doesn't prove it.

In his speech that follows, Antony proves Caesar wasn't ambitious, and thus turns the crowd into a mob.

You can find this speech in Act III, Scene 2 of the play.  The basic reason that Brutus has for having conspired to kill Caesar is that Caesar was going to take too much power for himself.  Brutus believes he was going to establish himself as king.

For this reason, Brutus says Rome needed Caesar dead.  He says he loved Caesar, but loved Rome more.  He says that the Romans had a choice -- they could have Caesar alive, but then they would be his slaves.  Or they could have Caesar dead, but they would be free.

Marc Antony stands over the body of Marcus Brutus and states: Only Brutus killed Caesar for the good or Rome.  He is the only one who stayed true to himself.  This was a man. In William Shakespeare’s Julius Caesar, Brutus was approached by Cassius in Act 1 to join the conspiracy to kill Caesar.  Brutus told him that he would need to think it over.

In Act II, Brutus has not slept since his talk with Cassius.  In his soliloquy, he  is thinking over his reasoning for joining the  conspiracy;  however, the time is here and he must decide. In his speech, Brutus uses three analogies to explain his logical conclusion for joining the assassination plan.

Immediately, the audience knows that Brutus has decided to kill Caesar. He begins with the statement:  “It must be by his death…” Brutus admits that he does not agree with Cassius in his personal hatred toward Caesar. Brutus has never seen Caesar act with ill will toward anyone.   For the general good of Rome is the reason that Brutus says that he will assassinate Caesar. 

1st analogy

If Caesar were crowned the Emperor, how would that impact Rome? If a person sees a poisonous snake like an adder, he needs to avoid the serpent.  What if Caesar was made king, how would that impact him? Like the adder, it might put a sting in him that would impact those around him?

The abuse of greatness is when it disjoins 
Remorse from power, and, to speak truth of Caesar, 
I have not known when his affections sway'd
More than his reason.

2nd analogy

The ladder of success is the next argument that Brutus uses.  When a person works his way up the ladder, he needs people along the way to help him move from one step to the next.  The person hoping for success keeps his eye on the goal.  When he gets to the top, sometimes he forgets the people that have helped him.  When his fortunes have been increased, then he will no longer need the supporters.

3rd analogy

Caesar is like a snake in its egg.  As long as it is in the egg, it is not problem for anyone.  However, when it comes out of the shell, then it is dangerous.  If it hatched, it could strike a person; therefore, kill the snake in the egg.  It will never get the chance to kill someone. Let the assassins do the same thing.  Kill Caesar before he has a chance to abuse his power.

When the conspirators meet with Brutus, the plans are in place to assassinate Caesar. 

What are the main motives of the conspirators in Shakespeare's Julius Caesar?

At the end of Shakespeare's play, after the death of Brutus, Marc Antony, who would have no reason to praise his enemy, eulogizes Brutus, acknowledging that of all the conspirators he was the only one who had noble intentions:

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. (5.7.74-78)

The others, as Antony acknowledges, are envious. That Cassius is envious is glaringly obvious. In Act I, Scene 2, Cassius speaks to Brutus of Caesar,

...And this man
Is now become a god, and Cassius is
A wretched creature, and must bend his body
If Caesar carelessly but nod on him. (2.2.117-120)

He continues, describing a epileptic seizure that Caesar suffered, adding,

             Ye gods! It doth amaze me,
A man of such feeble temper should
So get the start of the majestic world,
And bear the palm alone. (2.2.134-137)

Continuing his motif of Caesar being given too much power, Cassius, nevertheless, also continues to demonstrate his envy as he likens Caesar to a Colossus, who stands over the "petty men" like Cassius and Brutus who will be controlled, he implies, by such a tyrant. Casca, also envious, reinforces for Cassius that Caesar is desirous of power as he describes how Caesar pretended to refuse the coronet [crown] that Antony offered him because he acted as though he were reluctant to relinquish it. 

Knowing that Brutus is not envious and is known for his noble nature, Cassius forges letters in order to convince Brutus that it would be honorable to assassinate Caesar. So, he tricks Brutus in order to serve his own desires.

What were Cassius' motives for assassinating Caesar?

Cassius's motivations in killing Caesar are actually quite simplistic. To understand his motivations, it is important to look at two separate passages of the play. When Cassius lures Brutus into becoming an accomplice in the assassination, he cleverly paints Caesar as a power-hungry monarch who must be stopped. He claims that only men like himself and Brutus are able to stop Caesar, cleverly placing the blame for Caesar's rule not on Caesar himself but on Rome's citizens:

Men at some time are masters of their fates
The fault, dear Brutus, is not in our stars,
But in ourselves, that we are underlings

It seems unlikely the Cassius actually sees Caesar as a tyrant. In fact, Cassius simply seems envious of Caesar's rule. He seems to consider Caesar as unworthy of the throne and as an inferior man to himself. In an earlier passage in the play, Cassius recalls an event from his and Caesar's childhood, in which Caesar nearly drowned:

Caesar cried "Help me, Cassius, or I sink!"
I, as Aeneas, our great ancestor,
Did from the flames of Troy upon his shoulder
The old Anchises bear, so from the waves of Tiber
Did I the tired Caesar. And this man
Is now become a god, and Cassius is
A wretched creature and must bend his body
If Caesar carelessly but nod on him.

Cassius rescued Caesar from his deadly fate and is now wholly bitter that Caesar serves as the ruler. His motivations are simple and perhaps quite childish. He doesn't think Caesar is a tyrant, and he doesn't necessarily believe that Rome is suffering under Caesar. He simply thinks that he, Cassius, deserves the throne more. He craves the power and is envious of Caesar's success. He remembers the days of their youth in which he mercifully saved Caesar from death, and Cassius is disturbed to realize he must now bow down and worship this former boy who nearly drowned.

What motivated Cassius and Brutus to kill Caesar?

Two different motives, I think. Cassius seems pent-up with personal aggression towards Caesar, and real envy of how powerful he has become:

He had a fever when he was in Spain...
Ye gods! It doth amaze me
A man of such a feeble temper should
So get the start of the majestic world
And bear the palm alone.

He persuades Brutus by giving examples of Caesar's weakness, and employing extraordinary similes ("like a colossus...") to emphasise that Caesar has become extremely, extremely powerful. Cassius himself though, is never really presented as having honourable motives. He's serving his own interest, and his own dislike of Caesar: even Caesar himself says that men like Cassius "are never at heart's ease / When they behold a greater than themselves".

Brutus is made to seem much more noble and honourable. He himself claims that it's "not because I loved Caesar less, but because I loved Rome more". Yet Brutus is also hugely self-regarding and Cassius knows where to get him:

Brutus, and Caesar: what should be in that Caesar?
Why should that name be sounded more than yours?

And Brutus' response also refers to Brutus in the third person, as well as emphasising his reputation:

Brutus had rather be a villager
Than to repute himself a son of Rome
Under these hard conditions...

So both are motivated by the good of Rome, and their own personal gain. Traditionally Cassius has been supposed the more self-serving. I think Brutus is.