Why did Montesquieu believe that the powers of republican government should be separated among different branches of government?
Montesquieu believed that power caused people to become corrupt that the Biblical principles that man wants to do good but often ends up doing evil is true.
Montesquieu believed that a person or group of people that obtained power over other people would eventually use that power for their own benefit and to the detriment of other people.
Montesquieu wrote that by separating the powers between different branches of government that no one branch would obtain absolute power. The separation of powers would check the power of one group or person by the power held in the other branches. The checks and balances would prevent the corruption that power always causes in those who have too much power.
Rosseau, Voltaire believed that people were inherently good and only corrupt governments prevented people from being good. Montesquieu believed that people though having the desire to do good were the cause of the corruption not the government.