What is the primary responsibility of citizens use of power in a democracy?

The purpose of any government is to prevent anybody else from taking control. The responsibility of a citizen is to control his government. That is why governments always eventually treat citizens as enemies of the state.

It is rather rare for a citizen to figure that out. In most cases, citizens assume the government exists to protect them from competition. That is to say, they really want the government to treat some other citizen as an enemy of the state, but to protect them.