Why did the Romans overthrow the last king

The Romans overthrew the last king of Rome because he was a tyrant. There was a rebellion and it was decided to established a Republic (the first republic). Its aim was to prevent power to become concentrated in the hands of one man to eliminate the risk of a return to tyranny. This was done by creating two annually elected consuls as heads of the city and the army. Having to men in charge meant that the two would counterbalance each other.