Rather than take the honest title of "King", Mighty warlord Augustus had the Roman senate vote him the powers of several offices at once that would usually be held by several people. These titles brought with them titles including the military title Imperator (latin for "Your Royal Highness") whence we get the english word "Emperor" and Princeps which meant "First Citizen" (ie King). Augustus found that holding all these offices made his arms tired and so he gave the senate some responsibilities back such as responsibility for scheming, plotting and conspiring to kill. He also created the Praetorian Guard who were a bunch of hard bastards who made sure the senate didn't always carry out their responsibilities.