Aria watched as the poisonous fog started seeping inside everyone's home. She used her vision to watch how people started inhaling the poison and died instantly. It was a quick and painless way to die, which wasn't Aria's style as she preferred torturous death, but this would save her some time because massive reanimation and turning them all into undead would take a long time for Aria.
'And I still have to make sure their bodies stay intact until I can finish their reanimation. Decaying undead will not be useful,' Aria thought as she calculated everything inside her head.
Once these humans were successfully reanimated into undead, their bodies wouldn't decay until Aria let go of her spell and made them truly dead. Until that time, the decaying process would still continue.