Final answer:
Brutus joined the plot against Caesar because he, along with other senators, feared Caesar's indefinite dictatorship would turn the Republic into a tyranny. To Brutus, Caesar's growing power threatened the political traditions and liberties of Rome. Their fear of an autocratic rule inspired the assassination on the Ides of March.
Step-by-step explanation:
One of the reasons Brutus plotted against Julius Caesar was the belief that Caesar’s dictatorship posed a threat to the Roman Republic's democratic traditions. Caesar's continued accumulation of power, including his appointment as dictator for life, deeply concerned Brutus and many other Romans who feared the rise of an autocratic ruler. In Shakespeare's play Julius Caesar, Brutus argues that he is participating in the conspiracy against Caesar not out of envy or hatred, but out of love for Rome and its liberties. He expresses concern that Caesar, despite his virtues, might grow too powerful and potentially misuse that power, endangering the republic's future.
Suspicions regarding Caesar’s ambitions were further fuelled by Caesar’s actions prior to the conspiracy. His defiance of traditional Roman political mechanisms, appointing former enemies to government positions, and the subsequent offer of amnesty was seen by many, including Brutus, as tactics to ensure loyalty and consolidate his rule. The optimates, including Brutus, valued the republic's political structures, and thus, Caesar’s apparent attempt to override these systems propelled them into action, culminating in the infamous assassination on the Ides of March.