Lets assume this is true....
(I don't believe it)
How would they pick the 500 million to stay alive?
Where would these people be (protected), when the weapon will be fired (to kill the others)?
Will these 500 million people receive an "invitation card"?
"To avoid imminent death just come to our dinner-party located at these coordinates"
Or it will just fire randomly until the desired number will be reached?
What would happen with the world after that?
Would you not be allowed to have more than 1 or 2 children?
Cause if you have 3 then the population is increasing again.
Would they brain-wash all the people remained alive?
Otherwise I don't think many people would like to be told what to do by "them".
no, I don't see it working....