Ok. You obviously weren't completely blown back by the sheer indescribable awesomeness of a super weapon moon.
Imagine, little Sith Jr. being bullied incessantly by some guy from the planet of Utapau.
Now, he's all grown up, raging, seething and still a little sensitive at the Utapau dude. However, I believe, he is plagued by insecurities, cannot look at himself in the mirror without crying, and constantly doubts his own abilities in the Force. With all these emotional baggages he's carrying around, he can't very well bring himself to strike down his bully can he? Considering you've got to be face-to-face in order to bring about that dramatic moment with his overgrown torchlight whizzing through the air.
So how do we work around this?
We build a death star.
that's right sugar.
Instead of having deal with the psychological agony of meeting his tormentor face to face. He flicks a switch. NO. FORCE flicks a switch, the beams charges up and KAPAU! No more Utapau!
But coming back to the question. Where on earth did the dudes get the idea to get of all things. A moon with a laser on it?
The answer.
The Kugelbunker.
This was made by the Nazis in World War two to be used as a personal bunker by the individual soldier. The soldier would sit inside his glorified hamster ball and shoot from the little hole that was quite artistically cut out from the sphere. Think about the Allies' surprise when they realized that hamsters where shooting them with machine guns from concrete balls. Problematic.
Also, if you haven't already noticed, the startling similarities between both pictures, the Deathstar and the Kugelbunker. Imagine Lucas walking around in the museum, probably thinking about a wicked super weapon for his upcoming film with a cheesy title. Possibly getting bored with the "apius man" section on the first floor, he shuffles to the war section and behold! He spots a ball with a hole in it!
His mind starts to churn, and an equation starts to form.
Ball+hole+super weapon= ?
A flicker of genius. And the Deathstar was born.
this probably isn't true, but owh wells.