For the weekly upload, it was planned to gut the page without the very last panel. So it s not apparent right away, that the gun is empty. Up to that point, Barbera had no idea. She s very hard to intimidate, but somebody pulling the trigger startles even her. Nevertheless she gathered correctly that Angelo would have gone either for Gabriel or Jaune first - enough time in her mind to look for cover or kick his gun.
The other possibility was to cut the whole last line - but the time between the menacing gab + barbera panel and Angelo pulling the trigger is very short. Did not want this to be on the next page.
Well - why his gun was empty, you could already see in a previous panel.