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.
Brutus says he has to kill Caesar because the people are going to crown him King and with that role he would become too powerful and do damage to Rome. He says he knows of many powerful men before who were fair and just until they were given too much power. Once they obtain arbitrary rule, they turn their back on their friends an countrymen.
Brutus professes that he has no personal grudge against Caesar and in fact thinks of him as a good friend, but he sees Caesar's potential danger to Rome as reason enough to kill him which is evidenced in his famous simile in lines 32-34: "And therefore think him as a serpent's egg - Which, hatched, would as his kind grow mischieveous - and kill him in the shell.
Therefore, he must kill him, not because of any personal animostiy, but for the good of Rome.
he doesnt want him to be crown king because he will become more powerful and do damage to rome.