The Roman Republic, although the accounts of this era are semi-legendary at best, was supposed to have been founded after Tarquinius Superbus was ejected from Rome. His reign was productive in some aspects but was nonetheless filled with abuses, particularly against the senators: he marginalized them by refusing to consult them, and tried to reduce their numbers as best he could. He also became tyrannical in his administration of justice, trying capital cases by himself, without counselors, and using this as a way of stamping out opposition. This was capped off by his son's rape of a virtuous noblewoman named Lucretia, which caused Lucius Junius Brutus (the king's nephew, who had survived in Tarquin's regime by pretending to be slow-witted and thus non-threatening) to vow revenge. He summoned the people and inflamed them against Tarquinius, causing the assembly to strip the king of his imperium, the power of command and punishment that kings enjoyed. After Tarquinius was exiled, the Romans used existing voting procedures to select two magistrates (called praetors at the time, but they'd later be termed consuls, and this is how we know them today) and divided the power of imperium between them, so that no man would concentrate enough power in his own hands to tyrannize the Romans again.

Long story short, Tarquinius and his son were so outrageously oppressive (although reading between the lines, it's hard not to conclude that he pissed off the Senate more than the people generally, and that the revolution was motivated primarily by the aristocrats seeking to restore their position) that they completely soiled the concept of kingship for Romans, and the "Romans are free men, we don't need or want a king, kings are tyrants," message was passed down for centuries, with the result that Romans would be perpetually suspicious of people who accumulated too much power.