If the Empire was in complete control, there wouldn't be a Rebellion right? It's impossible to have complete control, and the the Emperor knew this. So, he built the biggest damn gun he could. If the Death Star hadn't been destroyed, it could have entered a star system, and by it's mere prescense alone, control a planet.
In the old Star Wars strategy game Rebellion, if you built a Death Star, and moved it into a system, it automatically fell under your control for manufacturing, resources, etc. However, if you ever used it to destroy a planet, systems would begin to turn against you faster. The first time I built one in the game, I immediatly jumped to Endor and "test fired" the gun.
