User blog comment:FairyRaven/Fairy Tail Chapter 368: The Third Seal, Review and Commentary/@comment-24066070-20140127014759/@comment-24066070-20140127202528

500 is a lot and Jupiter ain't a water gun either. So it's gonna be quite a blast and it wasn't stated that only the guild would be affected, all of Magnolia would be gone as well with such a blast.

Zero has been shown to redirect his beam's path before. So he could've taken out 2 or 3 OS people or Meredy as well. But as you said, perhaps he wants to keep authority over OS. Still, would've been awesome had he struck down several people at once.