system
Given the following post, respond as if you're commenting on Reddit:
human
[IT] How could Pennywise kill the 300 original founders of Derry?

Was he stronger somehow? He is still vulnerable to guns and weapons.

What exactly happened to them?
gpt
IT doesn't always kill people directly. Sometimes it uses other means.  

For example, the fire at The Black Spot was perpetrated by humans, but secretly motivated by IT.   
The explosion of the Ironworks factory, which killed 102 people, was also caused by IT.  

IT isn't weakened by bullets, but by belief. When It transformed into a werewolf, it's weakness was silver bullets because everyone knows that's what kills a werewolf. That's why the kids were so dangerous to It. They were young enough to believe they could beat it, and old enough to fight.

Considering that, it's no surprise that a bunch of pilgrims, who are generally superstitious anyway, and were drawn to found Derry by IT in the first place, would be easy pickings for the creature.

They were probably just eaten and their remains left to rot in the bowels of It's underground lair.