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.