Why didn't they just stay in the market?
I think that David and his group would have been better off just staying put in the market. Their whole reason and motivation to leave was to get away from Mrs. Carmody. Well, she was dead and not a threat anymore.
You might say, "well, her followers may have tried to avenge her." That is unlikely.
First up, Bud Brown (the store manager) was an unbeliever, yet when he ran back to the store, the congregation let him back in without a moment of hesitation.
Plus, you all saw the way the group ran for cover after Ollie shot her the first time. If they were truly devoted followers, they would have thrown themselves in front of her or rushed at Ollie and tried to disarm him.
One example of this is that in the seconds before shooting her the second time, Ollie walked up to her and Jim Grondin, one of her most faithful followers, was standing literally about three feet away, yet he did nothing to try to stop Ollie or help her.
Chances are that her group was only following her out of fear and returned to their sanity after she was killed. They likely would not have tried anything and if they did, all Ollie had to do was threaten or shoot one of them or maybe even take a hostage.
"Well, what about the monsters outside?"
Simple. Don't attract any attention. Stay away from the front windows and don't make any loud noises.
That's my opinion anyway. They should have just stayed there.