Final answer:
Brutus thought Caesar should not rule because Caesar's permanent dictatorship and reforms weakened the Senate and appeared tyrannical, betraying the Roman Republic's democratic traditions.
Step-by-step explanation:
Brutus believed Julius Caesar should not be the ruler of Rome because he saw Caesar's actions as a threat to the Republic's democratic principles and political balance. First, Caesar's indefinite dictatorship, a sharp departure from the Roman tradition of a temporary appointment, signaled an alarming consolidation of power which historically Romans distrusted due to their experience with autocratic Etruscan rule.
Caesar's reforms, like term limits for governors, further weakened the influence of the Senate, infuriating the aristocracy and widening class divisions. His shrewdness and opportunism in Roman politics, seen as self-serving and contemptuous of traditional alliances and friendships, fostered a sense of betrayal and enmity among peers such as Brutus.
Finally, Caesar's immunity from prosecution, due to holding military command, was threatened when the Senate refused to renew his command, which would potentially expose him to legal retribution for acts considered illegal by Republican standards, like waging unauthorized wars. The apprehension of Caesar imposing a tyrannical regime, common among the optimates and other senators, fed the conspiracy leading to his assassination in 44 BCE.